CVvjLrrrv\JtCGn-\_i LajLpri
THE
CHEMISTRY OF PAINTS AND PAINTING
THE
CHEMISTRY
OF
PAINTS AND PAINTING
BY
SIR ARTHUR H. CHURCH, K.C.V.O., F.R.S.
M.A., D.Sc, F.S.A.
Sometime Professor of Chemistry in the Royal Academy 0} Arts in London
FOURTH EDITION
REVISED AND ENLARGED
LONDON
SEELEY, SERVICE ^ CO. LIMITED
38 Great Russell Street
1915
1500
[Dedication oj the First and Second Editions.']
TO
SIR FREDERIC LEIGHTON, BART., P.R.A,,
WHO HAS ALWAYS SHOWN A DEEP INTEREST
IN ITS SUBJECT, AND HAS GREATLY
ENCOURAGED ITS AUTHOR,
THIS BOOK
IS, BY PERMISSION, DEDICATED.
s\^
r-v^ . ft
PREFACE
This handbook first appeared in the spring of i8go ; two
years afterwards a second and revised edition was pub.
lished. In igoi, when the work had been for some time
out of print, a thorough revision of its contents was
carried out, a good deal of new matter being introduced,
while a few pages, which had been occupied by a digest
of an important newspaper discussion on the effect of
light upon water-colour drawings, were not reprinted.
This account was omitted because it could no longer be
contended that many English water-colour drawings, ex-
posed to strong light for a considerable length of time,
had suffered no change in hue and depth.
Various additions and corrections have been made in
this, the fourth edition. A few of the paragraphs relating
to rather recondite subjects have been abridged or even
omitted. Indeed, an attempt has been made, in carrying
out the present revision, to simplify, so far as possible,
the way in which the results, obtained by chemists in the
study of painters' materials, are presented to the artist
and the student of art.
In the year 1908 a German translation of the third
edition of this book was published in Munich. It was
viii PREFACE
prepared and edited by the distinguished scientist Dr.
Wilhelm Ostwald. I have incorporated with the present
issue the substance of the paragraphs which he introduced
into my original text ; these are indicated by the sign 1[.
In preparing the following pages for the press, I have
to acknowledge, as on previous occasions, the help of
several friends and correspondents. Amongst these I
specially name Mr. J. Scott Taylor, many of whose
suggestions have been incorporated in the text, and also
Dr. A. P. Laurie, my successor in the chair of Chemistry
in the Royal Academy of Arts.
Of recent years the literature dealing with the subjects
to which the present handbook is devoted has greatly
increased. Several of the volumes named in my
* Bibliographical Notes ' are of sterling merit and contain
original material of no little importance. But I am
bound to confess that I have met with several disappoint-
ments when searching for records of new facts in recent
dictionary articles, reports of lectures, and treatises. On
perusal a familiar note seemed sometimes to be struck ;
and I ultimately identified not a little of the material as
my own. I will not dwell on this matter ; it is indeed
some consolation to feel that such transferences from my
pages would not have taken place had not the paragraphs
and tables and comments been deemed of some value.
But I trust that I myself shall not be thought guilty of
plagiarism because in 19 14 I reprint something, say a
table or a classification, which I published in 1890, but
which appeared ten years or more later as having been
PREFACE ix
devised by another chemist. Another reason for limiting
the number of books included in my list of titles is to be
found in the extensive Bibliography appended to Pro-
fessor Laurie's ' The Materials of the Painter's Craft.'
No very great differences will be found between the
present and the preceding edition, but I have endeavoured,
not only to introduce new matter which I think of import-
ance, but to simplify and make more exact the treatment
of the various subjects discussed ; and, in fine, to carry out
more fully the plan set forth in the Preface (here re-
printed) to the First Edition.
Arthur H. Church.
Kew Gardens,
jfune 2, 1914.
PREFACE TO THE FIRST EDITION
In the present volume the materials and methods of the
painter of pictures are viewed mainly from the chemical
standpoint. An attempt has been made to treat in orderly
sequence the various kinds of painting-grounds, the con-
stituents of vehicles and varnishes, the pigments them-
selves, and the chief processes of painting. Although
the artistic side of the numerous problems discussed has
not been neglected, the book is in no way intended to
teach manipulation to art students. It has been written
^^dth the view of explaining to artists, whether they be
accomplished masters or commencing students, the chief
chemical and physical characters of the materials with
which they deal and of the operations they practise. In
many instances a sketch of the processes for preparing
certain pigments and varnishes is given, not in order to
turn the painter into a colour-maker or a varnish-manu-
facturer, but rather that he may acquire a clearer insight
into the nature and properties of the most important con-
stituents entering into the composition of his pictures.
With regard, however, to the tests for purity and
genuineness which I have described in the following pages,
xii PREFACE TO THE FIRST EDITION
my object in introducing them has been different, for I trust
that (in some cases, at least) the easy experiments I have
recorded will be tried, especially with suspicious pigments.
The operations require but little time; the pieces of ap-
paratus needed, like the chemicals, are few and inexpen-
sive. And when the ease with which these testings can be
made has been proved by practice, the experimenter may
perhaps be induced to proceed a little farther, preparing his
own siccative oil, selecting and purifying his spirit of tur-
pentine, and so forth. That the painter should test the
varnishes he buys for hardness and toughness, and the pig-
ments for durability, may, I hope, be taken for granted.
' Titian managed pretty well without chemistry, did he
not ?' A distinguished artist asked me this question the
other day. But not only were the conditions under which
the painters of Titian's time worked simpler than those of
the nineteenth century, but grounds, paints, oils, and var-
nishes were generally prepared in the studios of the artists,
and under their own superintendence, so that the chances
of going wrong were comparatively limited. Audit is not
to be denied that a better acquaintance with the nature of
the materials which many of the old masters employed
would have caused their works to be handed down in
sounder preservation to future generations.
It is possible — I hope, indeed, it is probable — that this
book may be found of service to students who are purpos-
ing to devote themselves to certain manufacturing and
technical pursuits. I am aware that to those who refer to
PREFACE TO THE FIRST EDITION xiii
its pages for the revelation of all the secrets of colour-
manufacturers it may prove in some measure disappoint-
ing ; yet I trust that, in the way of information and
suggestion, the study of this volume will not be unattended
with advantage. It must be remembered that it is con-
fessedly an elementary manual only, written with a defi-
nite aim, but covering a very wide area of inquiry. And if
chemists should conclude that it contains too little chem-
istry, artists may perhaps think that it contains too much.
There are repetitions in the following pages, for the
topics discussed in some of the chapters overlap one
another. I am perfectly aware of having made the same
statement, given the same figure, and expressed the same
opinion in more than one place. The scheme of the work
required such repetitions. I felt sure that many an artist
or student would turn to one section or other of the book
without caring to read the whole. One inquirer would
like to ascertain at once what pigments were safe, what
dubious, what fugitive, by a reference to the tables in
Chapters XXI. and XXII. ; while another, anxious to learn
something of the evidence on which the several verdicts of
approval or condemnation were based, would expect to
find his requirements met in the pages devoted to trials of
pigments. Again, under the names of the individual pig-
ments, discussed in Chapters XIII. to XIX., some of the
changes described in the last part of the work are quoted.
Thus it happens that there are some materials common to
all of those sections of the book just named.
xiv PREFACE TO THE FIRST EDITION
Much of the substance of the lectures which I have
delivered before the Royal Academy since the year 1880
has been incorporated with the present manual, but it is
necessary to state that some of the original material to be
found in the following pages has been long before the
artistic world, and has found its way into the books and
essays of other writers. I say this, not for the purpose of
making reclamations of priority, but in order to prevent
myself from being charged with plagiarism. For instance,
so long ago as 1859, I described, for the first time, some of
the artistic uses of solid paraffin in a paper on the pro-
cesses of painting, read before the Oxford Architectural
Society; further details were given in a lecture to the
Architectural Association in 1862. On many other matters
connected with the chemistry of paints and painting, new
investigations and studies were published by me between
the years 1867 3-^^ 1872, particularly in notes and essays
entitled ' Chemical Aids to Art,' and ' The Chemistry of
the Fine Arts.' But my statements and results, whether
contained in the above publications or in my Academy
lectures, have not been, in all instances, referred to their
source, or reproduced with accuracy, while some have
been overlooked or forgotten.
In preparing the present volume I have made consider-
able use of several of the works named in my Biblio-
graphical Notes; I have consulted also the standard
chemical dictionaries of Watts and of Wurtz, the treatise
by Roscoe and Schorlemmer, besides many special papers
PREFACE TO THE FIRST EDITION xv
by other chemists. I wish I could have given an au-
thority for every statement not derived from my personal
experience, but in an elementary manual treating of
many diverse topics such a plan, even if it could have
been carried out, would have embarrassed my story with
a multitude of perplexing references.
I do not know of any one text- book which covers the
same ground as the volume now offered to the public.
Several small books on pigments — the most important of
all the materials employed by the artist — have indeed been
lately published, but the chemical information they afford
is generally meagre, and sometimes far from exact. One
recent little brochure, which lies before me, has, I confess,
caused me some amusement not wholly unshaded with
regret. The writer does not pose as a humourist, yet he
tells us, when we test for lead in cadmium red, first to
mix the sample with white lead before applying the usual
test for that metal. Chinese vermilion, he informs us, is
sulphide of arsenic, though it is really sulphide of mercury.
The presence of sulphides of baryta and lime is stated in
one place to lend a softness to the chromates of lead ; as
these sulphides instantly blacken these brilliant chromates,
perhaps they may be said to soften them. Coeruleum, a
stannate of cobalt, is directed to be made of carbonate of
soda, powdered flint, and oxide of copper, its two essential
constituents, the oxides of tin and cobalt, not being named.
These and many other equally preposterous statements
and directions may afford merriment to the chemist, but
xvi PREFACE TO THE FIRST EDITION
it is indeed pitiable that such teaching should be seriously
offered to artists and art-students.
It is satisfactory to know that several accomplished
chemists are now devoting themselves to the practical
study and improvement of pigments. Mr. A. P. Laurie,
Mr. H. Seward, and Mr. J. Scott Taylor, are all doing
good work in this direction.
It remains for me to express the hope that the readers
of this volume will favour me with any material at their
disposal which may serve for the correction and improve-
ment of its pages. I am aware of having omitted to
notice many interesting matters ; amongst these I include
certain pigments, derived from coal-tar products, which
have not yet been sufficiently tested. Then, too, the
materials and methods of ceramic and glass painting have
been excluded from consideration, mainly because their
adequate treatment, while demanding much space, would
have appealed to a comparatively limited group of
students.
If painters and chemists will grant me their help, I
trust that I may further justify ;, by means of an improved
edition of my book, the favourable reception which I hope
may be accorded to the first.
A. H. Church.
Kew, March, 1890.
BIBLIOGRAPHICAL NOTES
Blockx, J., ' Peinture a I'Huile.' Pp. iv, 98. Gand,
1881.
Cennini, Cennino, 'The Book of the Art.' Translated
into EngHsh by Mrs. C. J. Herringham, with
Notes on Mediaeval Methods by the Translator.
Pp. xxxviii, 288. London, 1899.
Church-Ostwald, ' Farben und Malerei.' Pp. xii, 376.
Miinchen, 1908.
Eastlake, Sir C. L., 'Materials for a History of Oil-
Painting.' Series I., pp. xii, 561 ; Series II., pp. xv,
432. London, 1847, 1869.
Field, G., ' Chromatography,' modernized by J. Scott
Taylor. Pp. viii, 207. Second edition. London,
1885.
Hurst, G. H., ' Painters' Colours, Oils, and Varnishes.'
Fifth edition, revised by Noel Heaton. Pp. xii, 528.
London, 191 3.
Jametel, Maurice, ' L'Encre de Chine.' Pp. xxx, 94.
Paris, 1882.
Lapparent, P. de, * Les Alterations des Couleurs.'
Pp. 36. Paris [1900].
Laurie, A. P., * The Materials of the Painter's Craft in
Europe and Egypt.' Pp. xvi, 444. London and
Edinburgh, 1910.
xviii BIBLIOGRAPHICAL NOTES
Laurie, A. P., ' The Pigments and Mediums of the Old
Masters.' Pp. xvi, 192. London, 19 14.
Moreau-Vauthier, Ch., < La Peinture.' Pp. xii, 322.
Paris, 191 2.
Ostwald, W., ' Letters to a Painter.' Translated from
the German by H. W. Morse. Pp. viii, 162.
Boston, U.S.A., 1906.
Parry, Ernest J., and Coste, John H., *The Chemistry
of Pigments.' Pp. viii, 280. London, 1902.
Russell, W. J., and Abney, W. de W., 'Action of Light
on Water-Colours ' (Blue-Book). Pp. 78. Appendix
of 14 diagrams. London, 1888.
Solomon, Solomon J., * The Practice of Oil-Painting.'
Pp. xvi, 278. London, 1910.
Vasari, Giorgio, ' On Technique.' Translated by Louisa
S. Maclehose ; edited and annotated by Professor
G. Baldwin Brown. Pp. xxiv, 328. London, 1907.
CONTENTS
PAGB
INTilODUCTION - - - - - I
PART I
PAINTING-GROUNDS
CHAPTER
I. PAPER, VELLUM, IVORY - - - - 7
II. PLASTER, GESSO, STONE, SLATE, ETC. - - 18
III. PANEL - - - - - - 29
IV. CANVAS - - - - - '34
PART II
VEHICLES AND VARNISHES
V. OILS - - - - - - 45
VI. RESINS, WAXES, AND SOLID PARAFFINS - 68
VII. YOLK AND WHITE OF EGG ; SIZE ; GLUE - 84
VIII. GUM, STARCH, DEXTRIN, HONEY, AND GLYCERIN 9 1
IX. WATER-GLASS, LIME- AND BARYTA-WATER - lOO
X. SOLVENTS AND DILUENTS - - - I06
XI. SICCATIVES OR DRYERS - - - " 125
XII. VARNISHES AND VEHICLES - - - I30
XX CONTENTS
PART III
PIGMENTS
CHAPTER PAGE
XIII. WHITE PIGMENTS . . . . 14^
XIV. YELLOW PIGMENTS - - - "157
XV. RED PIGMENTS - - - - 1 86
XVI. GREEN PIGMENTS - - - - 212
XVII. BLUE PIGMENTS - _ - - 226
XVIII. BROV^N PIGMENTS - - - - 252
XIX. BLACK PIGMENTS - - - _ 264
XX. CLASSIFICATION OF PIGMENTS - " "74
XXI. TABLES OF PERMANENT, FUGITIVE, AND ALTER-
ABLE PIGMENTS - - - - 283
XXII. SELECTED AND RESTRICTED PALETTES - 290
PART IV
METHODS AND RESULTS
XXIIL PAINTING-METHODS - - - " 3©!
XXIV. THE STUDY OF OLD PAINTINGS AND DRAWINGS 325
XXV. CONSERVATION OF PICTURES AND DRAWINGS - 342
XXVI. TRIALS OF PIGMENTS - - - "359
INDEX ----- 383
THE CHEMISTRY
OF PAINTS AND PAINTING
INTRODUCTION
The materials employed by ' picture-makers' are now very
numerous. Some of the old pigments, and painting-grounds,
and methods, have indeed fallen more or less completely
into disuse ; but, on the other hand, many new products,
both natural and artificial, have been added to the resources
of the artist, while several new processes of painting have
been introduced, or old methods modified. Nowadays it is
very seldom that a painter prepares for himself any one of
the materials which he uses, generally accepting, without
much hesitation and without examination, the paper, the
canvas, the paints, the oils, and the varnishes which his
colourman supplies, provided they respond, at first sight,
to his requirements. True he has abandoned, not without
regret, several of the mosttreacherous compounds by which
his immediate predecessors were seduced. ' Pure scarlet '
he has given up ; he is shy of asphalt ; tobacco-juice and
Spanish liquorice are no longer regarded as desirable
water-colours. He may go so far as to reject chromate
of lead, but he still employs the pigment called chrome
green, or green cinnabar, for he does not know that the
2 INTRODUCTION
same chromate of lead enters largely into its composition ;
and he still thinks that madder yellow is a sound paint,
because it is called madder, while he rejects the yellow
lakes, which are derived from the same source. His
linseed oil is neither made from pure linseed, nor cold^
drawn ; his copal varnish may not have a particle of pure
copal in it ; but both are taken on trust. I do not expect
that artists should become chemists trained to test their
materials, but they will place themselves in a position of
comparative security by acquiring an elementary know-
ledge of the origin, the composition, and the character-
istics of the various products with which their works are
constructed. An architect is expected to recognise the
sound or unsound quality of the timber, the stone, the
brick, the iron, with which the edifice he designs is
constructed : why should the painter take everything on
trust ? The purchaser of a picture ought not to be dis-
tressed by doubts as to its stability. The concentration
of the artist's attention on the definitely artistic side of
his practice must, of course, be in no wise interfered
with, but time may still be found for the acquisition of
such knowledge of his materials as shall enable him to
discriminate between the good and the bad. He may
even try, with great advantage, a few simple experiments
— experiments performed in a few minutes with the
simplest apparatus, and with the most innocent of re-
agents. These are the more necessary now that painters
no longer buy their raw materials, or make their own
paints, and oils, and varnishes, or prepare their own can-
vases and panels. Before colourmen generally under-
took such work, early in the seventeenth century, painters
were eager after receipts, and, there can be no doubt,
were ignorant of reasons : there was little exact science
INTRODUCTION 3
underlying their art. Yet it would be unfair to the best
colourmen of the present day to assume that they do not
endeavour to provide, as far as possible, sound materials.
But they do not manufacture all they sell. They are not
paper-makers, nor, as a rule, are they manufacturers of
oils and varnishes. Many of the pigments they furnish
are not of their own make. If, for instance, you inquire
the source of the artificial ultramarine you purchase of
your colour man, you will find that it has probably been
made in a factory wholly devoted to the manufacture of
that pigment. The production of this material can
indeed be properly carried on only in special establish-
ments thoroughly equipped for a peculiar and difficult
work. In reality, this specialization ought to be, and
generally is, advantageous, but it renders the position of
the colourman somewhat difficult. He has to assume
responsibility for the soundness and genuineness of many
products of the history and preparation of which he
knows little or nothing.
This difficulty confronts him in many directions. I have
known cases in which importers or manufacturers' travel-
lers have offered to artists' colourmen speciously prepared
but spurious pigments, such as madder carmine and rose
madder made from artificial alizarin, ultramarine ash
containing not a particle of the native lapis-lazuli, and a
gold ochre owing its colour to a basic ferric sulphate
instead of a hydrate. Then, too, some of the original
localities of a few native earths, such as terre verte and
raw umber, are practically exhausted, and most of the
new sources yield products of inferior hue. Hence the
temptation to * exalt ' the hue of the commercial article
by some seductive though dangerous addition.
After these introductory observations, I may refer the
4 INTRODUCTION
reader to the table of ccntents for the plan of the present
book, and to the prefaces for the object with which it has
been prepared. I would add, here, only this one remark,
that the materials with w^hich a painting is constructed
are described in definite order, beginning with the
ground, then passing on to the medium and the pig-
ments, not omitting the final varnish, and finally closing
with a brief summary of methods of painting, and of the
experimental studies by means of w^hich the conclusions
given in the earlier portions of the volume have been
reached.
PART I
PAh\TING-GROUNDS
Chapter I.— Paper. Vellum, Ivory. Chapter II. — Plaster or Intonaco,
Stone, etc. Chapter III. — Panel. Chapter IV.— Canvas.
CHAPTER I
PAPER, VELLUM, IVORY
As paper is used as the painting-ground for the vast
majority of works executed in water-colours, and as
this method of painting offers but slight protection to the
pigments employed against hostile influences, it becomes
of the greatest importance to ascertain that no unneces-
sary elements of danger are introduced in the paper itself.
We will now proceed to consider briefly the sources and
constituents of drawing-paper.
Linen from the common flax (Linum usitatissimtim), and
in the form of white rags, should be the basis of the pulp
used in the making of sound drawing-paper. In actual
practice the cheaper and weaker fibre of cotton (seed-hairs
of Gossypium sp.) has almost entirely displaced flax,
although during recent years a successful attempt has been
made in England to produce a high grade of hand-made
drawing-paper almost wholly composed of linen. Other
vegetable fibres might, no doubt, be employed for this
purpose. Thus, Japanese paper, prepared from the bast-
fibres of the paper-mulberry {Broiissonetia papyrifera), were
it made less absorbent by the introduction of a sufliciency
of size, would probably become an efficient, strong, and
durable substitute for linen-paper ; but at present linen-
papers, cotton-papers, and papers made from a mixture
7
xviii BIBLIOGRAPHICAL NOTES
Laurie, A. P., ' The Pigments and Mediums of the Old
Masters.* Pp. xvi, 192. London, 1914.
Moreau-Vauthier, Ch., « La Peinture.' Pp. xii, 322.
Paris, 1912.
Ostwald, W., * Letters to a Painter.' Translated from
the German by H. W. Morse. Pp. viii, 162.
Boston, U.S.A., 1906.
Parry, Ernest J., and Coste, John H., *The Chemistry
of Pigments.' Pp. viii, 280. London, 1902.
Russell, W. J., and Abney, W. de W., 'Action of Light
on Water-Colours ' (Blue-Book). Pp. 78. Appendix
of 14 diagrams. London, 1888.
Solomon, Solomon J., < The Practice of Oil-Painting.'
Pp. xvi, 278. London, 1910.
Vasari, Giorgio, * On Technique.' Translated by Louisa
S. Maclehose; edited and annotated by Professor
G. Baldwin Brown. Pp. xxiv, 328. London, 1907.
CONTENTS
INTRODUCTION
PAGE
I
PART I
PAINTING-GROUNDS
CHAPTER
1. PAPER, VELLUM, IVORY -
II. PLASTER, GESSO, STONE, SLATE, ETC.
III. PANEL - - - -
IV. CANVAS - - - -
7
i8
29
34
PART II
VEHICLES AND VARNISHES
V. OILS - - - - - - 45
VI. RESINS, WAXES, AND SOLID PARAFFINS - 68
VII. YOLK AND WHITE OF EGG ', SIZE ; GLUE - 84
VIII. GUM, STARCH, DEXTRIN, HONEY, AND GLYCERIN 9!
IX. WATER-GLASS, LIME- AND BARYTA- WATER - TOO
X. SOLVENTS AND DILUENTS - - - I06
XI. SICCATIVES OR DRYERS - - - * 1 25
Xn. VARNISHES AND VEHICLES - - - I30
xix
XX CONTENTS
PART III
PIGMENTS
CHAPTER PAGE
XIII. WHITE PIGMENTS - - - _ 14^
XIV. YELLOW PIGMENTS - - - "157
XV. RED PIGMENTS - - - - 1 86
XVI. GREEN PIGMENTS - - - - 212
XVII. BLUE PIGMENTS - _ . . 226
XVIII. BROWN PIGMENTS - - - - 252
XIX. BLACK PIGMENTS - - - _ 264
XX. CLASSIFICATION OF PIGMENTS - " -74
XXI. TABLES OF PERMANENT, FUGITIVE, AND ALTER-
ABLE PIGMENTS - - - - 283
XXII. SELECTED AND RESTRICTED PALETTES - 290
PART IV
METHODS AND RESULTS
XXIII. PAINTING-METHODS . - - -
XXIV. THE STUDY OF OLD PAINTINGS AND DRAWINGS
XXV. CONSERVATION OF PICTURES AND DRAWINGS -
XXVI. TRIALS OF PIGMENTS - - - -
INDEX . . . . .
301
THE CHEMISTRY
OF PAINTS AND PAINTING
INTRODUCTION
The materials employed by * picture-makers' are now very
numerous. Some of the old pigments, andpainting-grounds,
and methods, have indeed fallen more or less completely
into disuse ; but, on the other hand, many new products,
both natural and artificial, have been added to the resources
of the artist, while several new processes of painting have
been introduced, or old methods modified. Nowadays it is
very seldom that a painter prepares for himself any one of
the materials which he uses, generally accepting, without
much hesitation and without examination, the paper, the
canvas, the paints, the oils, and the varnishes which his
colourman supplies, provided they respond, at first sight,
to his requirements. True he has abandoned, not without
regret, several of the mosttreacherous compounds by which
his immediate predecessors were seduced. ' Pure scarlet '
he has given up ; he is shy of asphalt ; tobacco-juice and
Spanish liquorice are no longer regarded as desirable
water-colours. He may go so far as to reject chromate
of lead, but he still employs the pigment called chrome
green, or green cinnabar, for he does not know that the
2 INTRODUCTION
same chromate of lead enters largely into its composition ;
and he still thinks that madder yellow is a sound paint,
because it is called madder, while he rejects the yellow
lakes, which are derived from the same source. His
linseed oil is neither made from pure linseed, nor cold-
drawn ; his copal varnish may not have a particle of pure
copal in it ; but both are taken on trust. I do not expect
that artists should become chemists trained to test their
materials, but they will place themselves in a position of
comparative security by acquiring an elementary know-
ledge of the origin, the composition, and the character-
istics of the various products with which their works are
constructed. An architect is expected to recognise the
sound or unsound quality of the timber, the stone, the
brick, the iron, with which the edifice he designs is
constructed : why should the painter take everything on
trust ? The purchaser of a picture ought not to be dis-
tressed by doubts as to its stability. The concentration
of the artist's attention on the definitely artistic side of
his practice must, of course, be in no wise interfered
with, but time may still be found for the acquisition of
such knowledge of his materials as shall enable him to
discriminate between the good and the bad. He may
even try, with great advantage, a few simple experiments
— experiments performed in a few minutes with the
simplest apparatus, and with the most innocent of re-
agents. These are the more necessary now that painters
no longer buy their raw materials, or make their own
paints, and oils, and varnishes, or prepare their own can-
vases and panels. Before colourmen generally under-
took such work, early in the seventeenth century, painters
were eager after receipts, and, there can be no doubt,
were ignorant of reasons : there was little exact science
INTRODUCTION 3
underlying their art. Yet it would be unfair to the best
colourmen of the present day to assume that they do not
endeavour to provide, as far as possible, sound materials.
But they do not manufacture all they sell. They are not
paper-makers, nor, as a rule, are they manufacturers of
oils and varnishes. Many of the pigments they furnish
are not of their own make. If, for instance, you inquire
the source of the artificial ultramarine you purchase of
your colourman, you will find that it has probably been
made in a factory wholly devoted to the manufacture of
that pigment. The production of this material can
indeed be properly carried on only in special establish-
ments thoroughly equipped for a peculiar and difficult
work. In reality, this specialization ought to be, and
generally is, advantageous, but it renders the position of
the colourman somewhat difficult. He has to assume
responsibility for the soundness and genuineness of many
products of the history and preparation of which he
knows little or nothing.
This difficulty confronts him in many directions. I have
known cases in which importers or manufacturers' travel-
lers have offered to artists' colourmen speciously prepared
but spurious pigments, such as madder carmine and rose
madder made from artificial alizarin, ultramarine ash
containing not a particle of the native lapis-lazuli, and a
gold ochre owing its colour to a basic ferric sulphate
instead of a hydrate. Then, too, some of the original
localities of a few native earths, such as terre verte and
raw umber, are practically exhausted, and most of the
new sources yield products of inferior hue. Hence the
temptation to ' exalt ' the hue of the commercial article
by some seductive though dangerous addition.
After these introductory observations, I may refer the
4 INTRODUCTION
reader to the table of contents for the plan of the present
book, and to the prefaces for the object with which it has
been prepared. I would add, here, only this one remark,
that the materials with which a painting is constructed
are described in definite order, beginning with the
ground, then passing on to the medium and the pig-
ments, not omitting the final varnish, and finally closing
with a brief summary of methods of painting, and of the
experimental studies by means of which the conclusions
given in the earlier portions of the volume have been
reached.
PART I
PA INTING-GRO UNDS
Chapter I. — Paper, Vellum, Ivory. Chapter II. — Plaster or Intonaco,
Stone, etc. Chapter III. — Panel. Chapter IV. — Canvas.
CHAPTER I
PAPER, VELLUM, IVORY
As paper is used as the painting-ground for the vast
majority of works executed in water-colours, and as
this method of painting offers but slight protection to the
pigments employed against hostile influences, it becomes
of the greatest importance to ascertain that no unneces-
sary elements of danger are introduced in the paper itself.
We will now proceed to consider briefly the sources and
constituents of drawing-paper.
Linen from the common flax (Linum usitatissimum), and
in the form of white rags, should be the basis of the pulp
used in the making of sound drawing-paper. In actual
practice the cheaper and weaker fibre of cotton (seed-hairs
of Gossypium sp.) has almost entirely displaced flax,
although during recent years a successful attempt has been
made in England to produce a high grade of hand-made
drawing-paper almost wholly composed of linen. Other
vegetable fibres might, no doubt, be employed for this
purpose. Thus, Japanese paper, prepared from the bast-
fibres of the paper-mulberry {Broussonetia papyrifera), were
it made less absorbent by the introduction of a sufficiency
of size, would probably become an efficient, strong, and
durable substitute for linen-paper ; but at present linen-
papers, cotton-papers, and papers made from a mixture
7
8 CONSTITUENTS OF PAPER
of these fibres, are the only kinds with which water-
colourists are practically concerned.
During his explorations of Chinese Turkestan, Sir Aurel
Stein recovered many examples of early manuscripts
written on felted vegetable fibre, that is, paper. In the
British Museum are two scraps of such paper, with
Chinese writing, which must be dated somewhere between
the years a.d. 25 and 220. They are the most ancient
specimens of paper known to exist in the world. But the
manufacture of linen-paper in Europe has not at present
been traced back farther than the second half of the twelfth
century. Mr. W. H. James Weale, formerly Keeper of
the Art Library in the Victoria and Albert Museum,
informed me that the two first paper-mills in France were
set going near Ambert, in the valley of the Valeyre, by
men who, during their captivity in the Holy Land, were
forced to work at the manufacture of paper at Damascus.
One of these French mills was called ' Damascus,' the
other ' Ascalon.' This was previous to the year 11 89.
To Mr. Weale I am also indebted for an opportunity of
examining two early specimens, obtained from the ' Regis-
tre des Revenus de I'^^veche du Puy.' As one of the
sheets contains contemporary entries of the year 1273 —
the other entries belonging to 1289 — these papers are, at
least, as early as the years named. Both papers present
the creamy hue, the translucency, and the gloss of vellum.
One hundred square inches of the earlier specimen weigh
127 grains ; of the later, 163. Both are heavily sized with
paste made from wheaten starch. The use of starch for
sizing paper has been revived of recent years, but animal
size or jelly is still extensively employed. Some paper is,
indeed, made from felted linen pulp alone without size ;
but it is blotting or filter paper, and is quite unfitted for
ANAL YSIS OF DRA WING -PA PER 9
water-colour work, for when a wash of pigment is passed
over it, the colouring matter and the water partially separ-
ate, while the outline of the brush-stroke is not preserved.
Before entering further into the question of what are
the essential and what the accidental and unnecessary con-
stituents of paper, I give the summarized results of six
analyses, which show the percentage proportions found in
good samples :
ANALYSES OF DRAWING-PAPERS
Water
Size
Ash
Fibre
Hodgkinson, 1869 -
68 -
4-6 .
II
■ 87-5
English, 1876 -
10-9
61 •
II
. 81 -9
Dutch, 1876 -
I I'D
4-8 .
0-9 ■
■ 83-3
Whatman. 1885 -
7*4 -
6-3 ■
i-i
. 85-2
Arnold, 1894 -
74 -
7-6 ■
■ 1-5
- 835
'O. W.,'1897
87 -
5'5 ■
17 ■
. 84-1
Water.— It should be noted that the percentages of
water shown in these analyses vary considerably by reason
of variations in the humidity, temperature, and pressure of
the atmosphere to which the different papers had been
exposed just before the analyses were made. There are,
however, slight permanent peculiarities in samples made
from different fibres or sized in different ways ; in conse-
quence the moisture-absorbing and moisture-retaining pro-
perties of different papers are not precisely identical under
identical atmospheric conditions. This hygroscopic mois-
ture does, indeed, vary inversely with the temperature, and
directly with the amount of water- vapour in the air ; it is
increased also by an increased barometric pressure. There
is no doubt that if it could be wholly excluded, the larger
number of changes which occur in the pigments of a water-
colour drawing would be prevented. It is most injuriously
active when a framed drawing is exposed to considerable
10 ASH OF PAPER
ranges of temperature. Under these conditions the
moisture of the paper is first partly turned into vapour,
then condensed on the glass, and, lastly, is re-absorbed by
the paper, and, for a time, especially by the pigments lying
on its surface. This temporary condensation of an excess
of moisture upon the coloured surface does much injury
before hygroscopic equilibrium is once more re-estab-
lished. Much less harm would accrue were the vapour-
ized water allowed to escape.
Size. — The size must be considered next. It may be
applied to the pulp or to the sheet, and may consist of
gelatine with a little alum, of colophony or rosin dissolved
in soda-lye, followed by treatment with alum or alum-cake.
Sometimes starch is used along with alum or alum-cake.
From good drawing-papers, which are sized in the sheet
with animal size, the greater part of the size may be ex-
tracted by means of boiling distilled water, the solution
being usually neutral or faintly acid, sometimes faintly
alkaline, to test-papers. Gelatine and starch, to the extent
of about 5 per cent, of the weight of the paper, are the
safest sizing materials.
Ash. — The ash or mineral matter in paper may be
derived from three sources, namely, traces of the original
mineral substances taken up by the flax plant from the
soil, and still remaining associated with the felted pulp; the
mineral matters, such as soda and alum, introduced with
the size; and, lastly, the mineral compounds used to whiten,
to weight, or to finish the paper, or in bleaching the fibre
and as * antichlors.' In common and adulterated papers
the ash greatly exceeds i per cent., twelve parts per hun-
dred of paper being no unusual proportion. This * filling '
may contain or consist of the following substances : kaolin
or china-clay, silicate of lime or 'pearl-hardening,' chalk or
FIBRE OF PAPER ii
whitening, lead-white, baryta white or * white dressing,'
artificial gypsum or ♦ satin-dressing,' and a mixture of alu-
minium hydrate with magnesium carbonate or with calcium
carbonate, known as * satin-finish * or * satin-white.' Other
substances which increase the amount of ash left when a
paper is burnt are blue colouring matters, introduced to
counteract the natural yellow tint of the pulp. These
include artificial ultramarine, smalt or cobalt blue, and
Prussian blue.
Fibre. — What is put down as fibre in the analyses of
paper previously cited, is a substance, or group of sub-
stances, to which the name of cellulose is given by
chemists. Cellulose consists of the three elements —
carbon, hydrogen, and oxygen ; it is, when pure, entirely
combustible, leaving no ash.
The source of this cellulose is by no means without in-
fluence on the durability, strength, and working quality of
drawing-paper. The fibres of linen and of cotton present
distinct differences of form and resistance to strain. When
working on a paper with a knife so as to develop high lights,
the water-colour painter soon discovers the weakness and
fluffiness of abraded cotton, while the clear-cut surfaces of
linen are equally obvious. Even in washing and in taking
out lights from a drawing by sponging and rubbing, the
superiority of linen-paper to cotton-paper is very marked;
in fact, papers into which a high proportion of the latter
fibre enters will not stand much worrying. The other
fibrous materials commonly forming the basis of ordinary
papers are, on one score or another, less desirable than
cotton. Nearly all of them require, in order to fit them for
paper-making, a very drastic treatment, which is liable to
leave behind it traces of injurious chemicals, or to yield
altered material of lessened strength and permanence.
12 PAPER-MAKING
Wood-pulp, esparto, and straw-pulp belong to this cate-
gory.
Paper-making. — The technology of paper-making cannot
be discussed here, but a few references to the chemicals
employed in the process of manufacture may be usefully
given at this point. Amongst these chemical substances,
one or more of which will have been introduced into the
fibrous basis of the paper or into the size may be named:
caustic soda and caustic lime ; chloride of lime, magnesium
hypochlorite, moist chlorine gas, and sulphuric acid ;
alum, aluminium chloride, and aluminium sulphate ;
sodium sulphite ; gelatin. Of course, it is possible to
cleanse and bleach the higher class of rags without
having recourse to any chemical treatment, but the
' souring ' with sulphuric acid and the employment of
some soda or sodium carbonate to remove grease are
usual ; while there is always a salt of aluminium present
in the size. Indeed, in the best and purest drawing-
papers, the alum, or its equivalent, is the one ingredient
upon which the chemist interested in painting will look
with suspicion. But the subject of the presence of chemi-
cals, injurious or innocuous, in the finished product of the
paper-mill may be relegated to the following paragraphs.
Paper-testing. — The simplest test of the suitability of
any sample of drawing-paper for water-colour work con-
sists in applying to its surface uniform and weak washes
of a chosen set of sensitive pigments. A sound standard
paper is taken for comparison ; this may be ' Whatman,'
but it should be first swilled in cold distilled water for
five minutes, and then hung up to dry. In applying this
test, a strip of the sample to be tested and one of the
standard paper should be laid side by side, and then the
several colour washes, made with distilled water, carried
TESTING PAPER 13
across both strips by means of a broad brush. The pig-
ments used may be French ultramarine, chrome yellow,
and carmine. Unless they are employed in very dilute
admixture, the changes produced by alum and other
chemicals will not be perceptible. There should be no
bleaching of the ultramarine or the carmine, or any
blueing of the latter, and no dulling of the chrome, even
after the lapse of a week from the date of the experiment.
Washes of tincture of azolitmin from litmus^ tincture of
dahlia flowers, and tincture of methyl-orange may be
similarly applied to paper-strips ; in this case it will prob-
ably be found that the two former tests will show an
acid reaction, and the methyl-orange a basic or alkaline
reaction. This seemingly strange result has been found to
arise from the presence of a derivative of the alum in the
size, namely, an aluminium sulphate which is acid to some
tests and basic to others. This point has been established
by the experiments* of Messrs. Cross and Bevan, Mr. C.
Beadle, and Drs. P. N. Evans and Quirin Wirtz, who have
proved that all the drawing-papers of well-known makers
which they have examined contained no free sulphuric
acid. Of course, the question remains, * How far, if at all,
is the basic aluminium sulphate in drawing-paper injurious
to sensitive pigments ?' This inquiry can, I think, be an-
swered by applying the colour-tests already described, not
only to the suspected papers themselves, but also to extracts
from them made with cold distilled water and also with hot.
Other useful tests are the following :
1. Burn 100 grains of paper to a white ash ; not more
than I '5 grains of incombustible residue should be found.
2. Extract 100 grains of paper repeatedly with boiling
• * Journal of the Society of Chemical Industry {1892), pp. 212, 213,
261.
14 TESTING PAPER
distilled water. The united watery extracts, evaporated
to dryness, should not amount to 8 grains.
3. If straw or esparto fibre be present in a paper, it will
become red when immersed in a boiling i per cent, solution
of aniline sulphate.
Attempts have been made to size paper with casein dis-
solved in ammonia, and also with ' viscose,' a modified
cellulose made out of the substance of the paper itself by
means of water, caustic soda, and carbon disulphide. At
present, however, gelatin-sizing holds its own. The neces-
sity of introducing alum, or an equivalent of some other
aluminium salt, into this size is its chief drawback, although
an animal product of the group to which gelatin belongs,
being prone to decomposition and to the attacks of micro-
scopic organisms, itself constitutes a source of danger.
Alum is used not merely as an antiseptic, but because it
exerts a peculiar liquefying effect upon the size. A little
alum solution added to gelatin solution increases its stiff-
ness, but further additions up to an easily ascertained point
make the solution more mobile. It is absolutely necessary
to keep the alum percentage low ; I found in a batch of
one well-known make of drawing-paper that exactly twice
as much alum had been employed as was necessary. My
remonstrance with the manufacturers had its due effect.
The roughness or smoothness of the surface of the paper,
or cardboard, is not without influence on the permanence of
water-colours. The pigments become less intimately asso-
ciated with the smooth surface of a hot-pressed paper than
with a comparatively rough natural surface. The rough
surface is, however, liable to wider and more rapid fluctua-
tions in the amount of hygroscopic moisture.
Some apparently sound papers deteriorate in strength
and tint on being kept. Such changes may occur even when
SIZE IN PAPER 15
pure linen rags have been used for the pulp ; they may be
generally traced to the disintegrating action on the fibre of
the chemical bleaching agents employed. The development
of rust-spots, when not due to the mount or backing of a
drawing, arises from the presence of small particles of
metallic iron from the machinery having become em-
bedded in the pulp. These particles appear grey, brown,
or black ; they may be detected by placing a drop of oxalic
acid solution on the suspected spot, allowing it to dry, and
then moistening the place with a drop of a freshly-pre-
pared solution of tannin. If the particle be iron an ink-
stain will be produced. However, some dark spots con-
sist of blackened grease, or of tar, or of the paper-fungus
{Myxotrichum chavtanim).
Naturally, there is a small quantity of oil or fat in
paper ; it varies from 3 to 5 parts in a thousand. The
difficulty experienced in immediately wetting a surface of
paper, caused by the presence of this trace of oil, may be
overcome by first washing the surface with distilled water
to which a drop or two of caustic ammonia has been added.
A solution of the natural mixture of alkaline organic salts,
known as oxgall, effects the same purpose. The use of
borax had better be avoided. It is always advisable to
wet the whole surface of the paper before beginning a
water-colour drawing. Thus any abrasions or defects of
the surface will become apparent.
As drawing-papers are sized in the sheet they occasion-
ally show a peculiar defect arising from the irregular dis-
tribution of the size. In such cases, when the surface is
scraped off, an absorbent layer of imperfectly sized pulp is
revealed beneath. When such paper is used for water-
colour painting the sinking-in and running of the pigments
produce disastrous results ; but it is easy to guard against
i6 VELLUM AND IVORY
accidents of this sort by previously scraping and colouring
a corner of the sheet to be used. The peculiarity is gener-
ally owing to the too prolonged and slow drying of the
sheets of paper after they have been removed from the
warm sizing-bath and pressed. The solution of size is
brought to the surfaces from the interior of the sheet, and
remains there. Moreover, in very slow drying, the size is
apt to decompose with loss of its glutinous character and,
possibly, the formation of mildew. A good drawing-paper
will indeed have rather more size at the surface than in
the interior, this result being secured by a rate of drying
which is neither too rapid nor too slow. Let us add that
the strength of paper when completely wetted and in the
presence of free water, is very low. If, however, it has
been gelatin-sized and afterwards sprayed with a 40 per
cent, solution of formalin to coagulate the gelatin it be-
comes appreciably stronger.
As to vellum, parchment, and ivory, little need be said.
All three contain the characteristic ingredient ossein, an
insoluble nitrogenous organic substance, which by long
boiling with water is converted into gelatin : a solution of
gelatin constitutes ordinary size. Water-colour paints
placed upon any of these materials sink either very
slightly, or not at all into their substance — a very few,
such as aureolin, strontia-yellow, and madder carmine,
stain the superficial layer. The old method of preparing
vellum for the reception of water-colours consisted in
rubbing the surface with very finely-ground bone-ash, or
with pulverized sandarac. Pumice-stone or cuttle-fish, re-
duced to a minutely divided state by pounding, grinding,
and sifting, may be used for this purpose ; the infusorial
earth known as polishing silica, or kieselguhv, may also be
employed.
VELLUM AND IVORY 17
Ivory which has become yellowish through age and se-
clusion from light may be safely bleached by contact with
an ethereal solution of hydrogen peroxide. The treatment
is best carried out in a wide-mouthed stoppered bottle,
care being taken to immerse the sheets of ivory wholly in
the liquid, and not to allow them to touch each other.
Much care is necessary in selecting tinted and coarse
coloured papers for water-colour work. The tints of the
former are often obtained by the introduction of fugitive
pigments into the pulp ; the latter are often made of in-
ferior and mixed fibres, and sometimes contain lead- white
and other m]\iy:\ous fillings. 'Turner' paper, for example,
owes its grey-blue tint to the presence of indigo, while
* Varley' paper contains about 20 per cent, of 'mechanical'
wood-pulp, a material which steadily darkens into brown
after but a short exposure to light. ' Sugar ' paper, what-
ever its hue, should be avoided. Mill-board is often
made of wood-pulp, oakum and straw-pulp : its surface is
primed for oil-painting in the same way as canvas.
CHAPTER II
PLASTER, GESSO, STONE, SLATE, ETC.
The painting-grounds to be considered in this chapter
consist mainly of mineral substances. However their con-
stituents may be varied, in accordance with the process to
be used in painting upon them, the wall or backing upon
which they are spread should fulfil certain conditions. It
must be naturally dry, free from soluble saline matters, and
not very porous. A damp-proof course above the level
of the ground is necessary, and the wall should be well-
built, and free from tremors. A double wall well-bonded
has been recommended : in this case the air enclosed
between its two divisions should not be stagnant.
Before being plastered, the wall, whether its surface be
of stone, bricks and mortar, roughened slate, or tiles, must
be thoroughly wetted with lime or baryta-water. The
plaster is applied in two or more coats, the coarsest and
thickest first. In the case of a ground for fresco the two
ingredients usually employed are (or rather were) pure
slaked lime, and clean sharp silicious sand. The sand
must be uniform in grain, white, and free from soluble
salts. The slaked lime is so important an ingredient in the
majority of plasters, that it is expedient to describe its
preparation once for all. Before doing so we may state
the relations subsisting between the three compounds
known generally as carbonate of lime (chalk), or mild Hme,
i8
COMPOUNDS OF LIME 19
burnt lime, or quicklime, and slaked lime. The first of
these is neutral and nearly insoluble in pure water, the
second and third are alkaline and caustic. When burnt
lime unites with water to form slaked lime it becomes
slightly soluble in pure water. In chemical language these
three compounds are called respectively calcium carbon-
ate, calcium oxide, calcium hydrate (or hydroxide). From
the first substance the others are readily obtained. If
calcium carbonate, often called carbonate of lime, be
heated to a sufficient temperature, it is decomposed, being
resolved into carbon dioxide (carbonic acid gas) which
escapes, and calcium oxide (lime) which remains : from
100 parts by weight of the carbonate 56 parts of lime, that
is, burnt lime, are obtained. Placed in water or exposed
to moist air this burnt lime combines with water, 56 parts
of it uniting with 18 parts of water to yield 74 parts of
slaked lime, calcium hydrate. In the ordinary country
atmosphere, which contains no more than 3 measures of
carbonic acid gas per 10,000, slaked lime or calcium
hydrate loses its combined water, slowly becoming once
more the carbonate from which it was originally produced :
74 parts of hydrate lose 18 parts of water and combine
with 44 parts of carbonic acid, and yield 100 parts of car-
bonate. Thus mild lime is formed once more from caustic
lime. By this change, if it be effected in the presence of
a sufficiency of free water — that is, if the hydrate of lime
be in the state of a firm paste — the whole substance
becomes a hard crystalline solid, like an opaque marble.
Advantage may be taken of this hardening or cementing
process to firmly incorporate other substances with the
lime, Silicious sand, infusorial earth, pumice, marble
powder, and many other mineral substances, may be thus
introduced. Such of these materials as are silicious may
20 LI ME- PUTTY
contain silica in a form which is known as ' soluble silica.'
This substance further strengthens the plaster by forming
with a part of the lime an insoluble compound called sili-
cate of lime. To return to the preparation for artistic
purposes of hydrate of lime. White or black marble,
limestone, chalk, or other fairly pure forms of carbonate
of lime are first of all hurnt, and then the quicklime pro-
duced is slaked with clean water. This is done in a
grouting box, having a sluice i or 2 inches from the bottom.
Run the thick cream of lime into a tank of slate and keep
it, covered loosely, for two months. At the end of this time
it will be ready for all the rougher purposes of plastering.
For finer work the grouting operation is to be repeated, and
the cream of lime strained through hair-sieves, and pre-
served in screw-top stoneware jars. Some water will accu-
mulate above the lime-putty, as it may be called, in these
jars ; it should be poured off or drawn off, from time to
time. The jars are kept tightly closed to prevent further
carbonation of the lime hydrate. This change, if carried
beyond a certain point, is undesirable, since the binding
and hardening powers of the lime would thereby be
lessened seriously, or even vanish altogether by its con-
version into mild lime : not more than one-third or at most
two-fifths of the lime should be converted into the car-
bonate. The lime-putty thus prepared may be used for
plaster and intonaco with the certainty that it will not give
rise to defects in the painting-grounds made therewith.
Much lime paste of this kind was prepared for the works
in fresco in the Houses of Parliament, and was kept in
the cellars under that building, where probably some of
it still remains. I have made many experiments with
samples from that source, and can speak with confidence
of its excellent quality.
FRESCO GROUNDS 21
Btion' Fresco. — A good mixture for the first application to
the moistened wall consists of 2 parts (by weight) of clean
sharp sand to one of lime-putty. When one or more coats
of this mixture have been duly laid and have set, then the
surface is ready to receive the final coat or intonaco, the
actual painting-ground. Before this is applied, the rougher
plaster below must be thoroughly wetted with distilled or
lime water. The sand in the intonaco is of finer and more
uniform grain than that previously employed; the intonaco
itself is only one eighth of an inch in thickness. All the
coats must be laid without having recourse to scraping or
* floating' ; the latter operation brings too much lime up to
the surface. Considerable practice and manual dexterity
are needed in these operations. The work of painting is
at once commenced when the intonaco has been laid, no
more being spread at one time than the artist can cover in
the day. Upon the wet soft plaster the cartoon is laid, and
the outlines and other important parts pounced in, trans-
ferred, or impressed by an ivory point. Rapidity and
firmness of execution, with the distribution of a uniform
thickness of pigment, are matters to which special atten-
tion must be paid. The chemistry of this method of
painting will be discussed in Chapter XXIII.
Many modifications in the preparation, proportions, and
materials of fresco painting-grounds have been introduced
or suggested from time to time. I have found the following
mixture to yield an excellent plaster for this purpose :
Three parts of burnt lime in very fine powder are ground
up with 2 parts of whitening or prepared chalk ; the mix-
ture is grouted, and then strained through hair-sieves ;
5 parts of the putty thus obtained are mixed with 5 parts
of sifted crushed marble, or with 5 parts of sharp, fine,
sifted sand, or with 3 parts of sifted pumice, or with the
22 TEMPERA GROUNDS
same quantity of infusorial (silicious) earth ; the whole
being moistened with a sufficient quantity of lime-water
to render working easy. For the undercoats the sand, etc.,
introduced may be coarser ; while a small quantity of the
most silky and whitest asbestos, cut with scissors into
short uniform lengths, will prove a desirable addition.
The asbestos * lessens the risk of any lack of continuity
in the undercoats.
Fresco-Secco and Tempera. — For fresco-secco the same
ground as that required for true fresco may be used, but it
is allowed time to dry and harden. So long as it contains
any caustic lime this ground is unfitted for work in tem-
pera, as its alkaline nature seriously limits the variety of
pigments which may be employed in this method. When
carbonation of the lime is complete it may be employed
for tempera-painting, the surface being first treated with
warm size. Many Greek and Byzantine paintings were,
however, executed upon a caustic lime ground, but the
pigments employed consisted chiefly of those natural
earths which are unaffected by alkalies. In these Greek
tempera-grounds slaked lime mixed with chopped straw,
flax, or cotton, formed the basis of the plaster. It is
scarcely necessary to remark that these vegetable
materials are liable to decay and to cause discoloration
of the ground.
The ordinary ground for Italian and Spanish tempera-
paintings consisted either of whitening and size, or of
burnt gypsum (that is, plaster of Paris), stirred well with
* Professor Laurie, in his ' Materials of the Painter's Craft '
(p. 138), attributes this recommendation as to the use of asbestos
to Mr. James Ward, who names it in his book on ' Fresco Painting '
(p. 14) published in 1909. But the present author published the
same recommendation with fuller instructions in the year 1890 : it
will be found on p. 18 of the first edition of the present handbook :
but he also may have been anticipated.
TEMPERA GROUNDS 23
water so as to lose the power of setting, strained, and
mixed with size. Sometimes both whitening and slaked
burnt gypsum are found together as constituents of the
ground. The ground was laid directly on the panel, or
on the cloth which had been previously glued to the
wood. Great care was taken by sifting and washing to
secure the fineness and purity of the whitening (calcium
carbonate) and of the slaked plaster of Paris (calcium
sulphate united with two proportions of water). Various
kinds of size were used ; one of the best was made partly
from parchment, partly from the finer kind of fish-glue.
An excess of size will cause the ground to crack ; it must
never contain such a quantity as to be rendered non-
absorbent. All tempera-grounds of gesso were originally
absorbent ; in course of time they have become more so
owing to the decay of the size. Whether they were
afterwards to be painted in tempera or oil they were
always first sized. This sizing preserved the luminous
whiteness of the ground, which was unable to absorb the
oil of oil-paints or that present in the egg-yolks employed
in tempera. A proof of the existence of this layer of size
above the ground proper is obtained in the process of
transferring old tempera and oil pictures to canvas, for
in such cases we find discoloration of the ground under
cracks only where both the size and the paint above it
have become fissured.
It will have been gathered from what has been stated
in the preceding paragraph that a non-caustic tempera-
ground is suitable for work in oils. In the latter case,
however, it must be perfectly dry before the painting is
commenced. It should be gently warmed and rubbed
with a little clean spirits of turpentine before laying on
the first coat of oil-paint.
24 STEREOCHROME GROUNDS
Steveochromy. — The ground for stereochromy has been
modified several times since the first introduction of this
method of water-glass painting. Originally it was recom-
mended to use an undercoat containing 2 parts of sharp
sand, 2 parts of fine sand, and i part of slaked lime in
fine powder. Upon this was laid an intonaco of one-tenth
to one-eighth of an inch in thickness, made of 3 parts of
fine sand and i part of slaked lime. The fineness or
coarseness of the sand in the intonaco must, however, be
regulated by the nature of the surface required by the
artist. I see that nearly fifty years ago I recommended*
the employment of sifted white marble powder, and of
several other substitutes for sand, recommendations
which, within the last few years, have been again brought
forward by Herr Adolph Keim. Oxide of zinc may be
advantageously substituted for a part of the lime in the
intonaco, and it may be added to the pigments. Keim
recommends the wall to be first coated with a mixture of
I part of burnt lime (which is to be slaked with distilled
water), and 4 parts of a composition consisting of coarse
quartz sand, infusorial earth, and powdered marble. The
actual painting-ground, which is from one-eighth to a
quarter of an inch in thickness, is made of i part of
slaked lime, and 8 parts of a mixture of the finest quartz
sand, marble sand, marble meal, and infusorial earth.
Fine asbestos paper, wetted with lime-water, and firmly
pressed by rolling into a soft freshly-laid lime and sand-
plaster, makes an excellent ground for stereochrome
painting ; but as a single breadth only of this paper can
be used, the size of the work that can be executed
upon it is somewhat limited. All the precautions as to
* 'Chemistry of the Fine Arts' in Cassell's 'Technical Edu-
cator.'
SPIRIT-FRESCO GROUNDS
25
dryness of the wall and purity of the materials, already
noted in the case of fresco-grounds, must be observed in
reference to those intended for stereochrome painting.
Spirit-Fresco. — The ground recommended by the late
Mr. Gambier Parry for that modified form of varnish-
painting to which he gave the name of * Spirit-Fresco ' is
identical with that required for true fresco. All the usual
precautions as to the dryness of the backing, and its
freedom from soluble salts, must be taken. The plaster
must be allowed to dry completely before the operation of
saturating it with the medium is commenced ; the lime in
it should also have become mild — that is, carbonated.
(See Chapter XXIII. for tests for alkalinity and moisture.)
Syringing the plaster with distilled water previously
charged under pressure with carbonic acid gas, though
it delays the drying, hastens the carbonation of the lime
materially. To complete the preparation of the ground,
it should, when quite dry, be soaked with a mixture of
two parts of the medium (Chapter XII.), and three of tur-
pentine. After two days, this treatment must be repeated.
A third application may be needed for very porous
grounds. Another period of forty-eight hours having
elapsed, the surface receives a coat of white paint, made
of equal parts of white lead and gilder's whitening,
ground up with the medium diluted with one-fourth or
one-third its bulk of turpentine. This priming is re-
peated when the first coat is dry. After three weeks, the
painting may be commenced. Stone and terra-cotta, if
sufficiently porous, may be primed in the same way as
plaster. Under no circumstances should cements con-
taining plaster of Paris be introduced into the grounds
used for spirit-fresco.
During the last twenty years a considerable number of
26 SLATE AND STONE
large mural paintings have been executed either in Gam-
bier Parry's medium or in the paraffin-copal medium.
Some of these works have been painted directly on
plastered walls, some on canvases which have been
afterwards affixed by marouflage to the surfaces prepared
to receive them. To the latter category belong nearly
all the paintings in the Ambulatory of the Royal
Exchange, London. Each of the compartments has
been very carefully arranged with a view to secure dry-
ness and freedom from soluble saline matter. In front
of the wall itself has been fixed a slate slab slightly
inclined forwards at the top and having a ventilated air-
space behind it. Upon the slate the finished picture has
been attached (or maroufle) by means of a thick paste of
white-lead, oil, and copal-varnish, spread not only upon the
slate, but simultaneously upon the back of the canvas. It
may be affirmed that paintings so secured are free from all
risk of injury from the back. In an atmosphere like that
of London the surface of the painting must either be pro-
tected by glass or be periodically cleansed from deposits
of dust, soot, tarry matters, and the other impurities
which are described in Chapter XXV. of this handbook.
Several fresh materials have been recently employed as
painting-grounds. They are either patent or secret pre-
parations, dependent in general for their solidification upon
reactions between insoluble earthy and alkaline earthy
matters, such as china-clay, asbestos, and compounds of
lime and magnesia, with solutions of such salts as mag-
nesium chloride, aluminium sulphate, and alum. There is
sometimes a lack of tenacity, and always a lack of tough-
ness in these mixtures, but some artists find them to pos-
sess precisely the texture and absorptive character they
desire in grounds not only for tempera, but also for oil-
SLATE AND STONE 27
painting, and they may be spread on canvas as well as on
more rigid supports. There is some danger of want of
adhesion between the paint and the ground. It is also
necessary to make sure that the materials of the ground
do not affect sensitive pigments such as ultramarine. The
hardening or petrifying liquids which in most cases are
used in association with solid preparations to make the
grounds in question, are invariably acid to test-paper,
unlike the alkaline silicates described in Chapter IX.
Slate may be used as the ground for spirit-fresco and oil-
painting ; but its freedom from crystals of iron-pyrites,
which present a brass-yellow colour, must be first ascer-
tained. The firm adhesion of any priming, or other layers
of oil-paint which may be applied subsequently, to slate
may be secured in the following manner. The slate is
slowly warmed in a water-oven, and thus becomes quite
dry. While still warm, it receives a very thin coat of
oil-copal varnish, largely diluted with turpentine or with
toluol, and applied warm. When this film is hard, the
painting may be carried out as in the ordinary way of
using oil-colours ; a priming of flake-white ground in oil
and mixed with a little copal- varnish and turpentine, may
be first applied, if desired. Terra-cotta and stone may be
treated in the same way, but, being more absorbent than
slate, the process recommended on p. 31 is preferable.
Owing to the presence of sulphuric acid in urban air
painting-grounds containing calcium carbonate are liable
to an injurious change, the carbonate being turned in
part into the hydrous sulphate (gypsum) with a consider-
able increase of bulk. Then, through such expansion,
the surface-pigment becomes fissured and even detached.
It will be readily understood that grounds consisting
chiefly of sulphate of lime are not susceptible of such
28 SLATE AND STONE
change. So, where damp can be excluded, they may be
used for mural paintings, ground flints or fine sand being
admixed with the burnt gypsum employed. The paint-
ings of the buried cities of Chinese Turkestan explored
by Sir Aurel Stein were executed on grounds of this kind
— grounds, that is, of nearly pure plaster of Paris.
CHAPTER III
PANEL
Wood, as a backing for the painting-ground of works in
tempera and oil, presents some advantages over plaster and
canvas. Its chief merit lies, perhaps, in its comparative
immunity from mechanical injuries. The wood selected
must be hard, that its surface may resist blows and abra-
sion ; and it must not contain much resin, gum, colouring-
matter, or other ' extractives,' as they are called, or else
discoloration of the painting-ground, or priming, may
occur. Wood grown in poor soils, in temperate climates,
and felled in winter, is the best. The Flemings used oak ;
the Italians white poplar. But oak often proves treacher-
ous, through irregular sh mkage; while poplar is too soft.
Italian painters employed, also, the wood of the stone
pine and chestnut. Leonardo da Vinci recommended
cypress, pear, and service-tree. Mahogany, which was
unknown to the old painters, is now generally employed.
Teak and cedar, and also American or black walnut,
deserve further trial.
The specific gravity of wood varies from 0-3 to 1-3 ; the
lighter kinds contain large volumes of interstitial air.
The longitudinal contraction of wood is much less than
the transverse ; the distribution, form, and number of the
cracks in old panel-pictures is often to be traced to this
cause.
29
30 PANEL
Wood contains (i) water, (2) ligno-cellulose, (3) extrac-
tives, (4) ash or mineral matter. The water, in thoroughly-
seasoned and air-dried wood, generally constitutes about
one-eighth part of its weight. The main constituent of
wood is the so-called ligno-cellulose, which is present to
the extent of from 75 to 85 per cent. It may be resolved
into two substances, which, for convenience' sake, are here
called cellulose and lignose. The extractives belong to
two groups — one, soluble in alcohol and ether, consists
chiefly of resins ; the other, soluble in cold or hot water,
or else in very dilute alkalies, includes tannin, albu-
minoids, gum, and colouring-matters. The following
analyses of three kinds of wood in an air-dried state will
convey a fair idea of their constitution in 100 parts :
Mahogany
Oak
Pine
Water -
■ 124 -
- 131 -
. 12-9
Cellulose
490 -
- 39-5 -
- 533
Lignose -
276 -
■ 343 -
- 28-2
Ash
i-i -
I '2 -
- 0-3
Resin
I'O -
- 0-9 -
- 1-6
Water-extract
8-9 -
- I I'O -
- 37
The preparation of panels for painting requires much
time and trouble. The directions given by ancient authori-
ties are numerous, and not always accordant. One author
tells us to boil the wood ; another says we are to coat it
with mastic dissolved in twice-distilled turpentine and
mixed with white. Then it is to be treated twice or thrice
with spirits of wine, in which some white arsenic or corro-
sive sublimate has been dissolved ; coats of boiled oil, of
liquid-varnish* and white, and of verdigris and yellow are
subsequently mentioned. Probably the best method of
treating the harder woods intended for pictures is, after
* Made by boiling i part of sandarac in 3 parts of linseed-oil.
PREPARATION OF PANELS 31
thorough seasoning, first of all to reduce the panel, by
planing and glass-papering both sides equally, to the desired
thickness. The panel is then soaked in water heated to
50° C, and then steamed. When dry, it receives a wash
on both sides of a solution of corrosive sublimate in
methylated spirit; it is again dried and seasoned in a warm
air-chamber. After these operations, the panel should not
require more than a slight rubbing with fine glass-paper,
in order to render both surfaces plane. For panels to be
used for oil-pictures, a priming is now applied, consisting
of white lead, a little copal- varnish, and drying linseed-
oil prepared by means of borate or oxalate of manganese.
Allow this coat, which is intended to fill up the cavities
and pores of the wood, to dry thoroughly, and then apply
another coat in the transverse direction; subsequent coats
should contain nothing but white lead (or other pigment)
and the drying oil. Repeated smoothings of each coat,
when hard, with fine pumice-powder are necessary; the
last coat may consist of zinc-white and drying-oil. Both
sides of the panel should be treated, as far as possible,
alike, so that they may be equally loaded, and equally
protected ; but the pumice-rubbings are, of course, not
required for the back of the panel. The object of priming
the back is twofold — the prevention of decay and of the
attacks of insects ; and the avoidance of that gradual cur-
vature whereby the protected front becomes convex, and
the unprotected back concave. This change occurs
through the slow loss of water from the back of the panel
— a loss which is generally accompanied by a loss of some
of the organic constituents of the wood through oxidation.
Here it may be mentioned that the original steaming of the
panel removes some of the extractives, and coagulates the
albuminoids present, which are generally the first cause
32 PREPARATION OF PANELS
of decay. This decay is not primarily a chemical and
spontaneous one, but is commenced by certain minute
organisms, the growth and increase of which is, in part,
dependent upon the presence of available albuminoids, but
which involves also the destruction of some of the other
extractives, and even of the ligno-cellulose itself. The
corrosive sublimate employed helps to sterilize the wood,
and to prevent the inroads of animal organisms.
In order to avoid the disastrous effects of transverse
shrinkage upon compound panels, the old painters glued
linen cloth, or vellum, or parchment, or tinfoil to the front
surface of the wood, and on this they spread their gesso or
painting-ground. Gesso, made of plaster of Paris and size,
or of whitening and size, often lost its cohesion through
the decay of the binding material, and in consequence
became fragile and powdery ; the panel itself decayed, and
thus at last the linen or parchment remained as the best
preserved element of the composite structure. Were we
to avoid gesso and use lead-primed canvas glued to panel,
we should really be painting upon canvas backed or pro-
tected by wood. Panel is to be recommended for modern
work only when a single piece of uniform and well-seasoned
wood of sufficient size can be secured. However, an
excellent cement for joining panels together was some-
times used with success. It consisted of lime and cheese,
both in fine powder, the latter having been grated, and
then washed with water. These materials intimately
mixed and then ground into a paste with water, yield a
tough and adhesive cement which becomes of rocky
hardness.
In order to prepare a panel for tempera work, it should
be treated in the manner above described, substituting
for the priming with oil, white lead and copal-varnish, a
PREPARATION OF PANELS 33
mixture consisting partly of parchment-size, partly of
fish-glue, and whitening.
It is very probable that some of the hard, fine-grained
woods of British India and of North Borneo will furnish
excellent materials for picture-panels. At present experi-
ments in this direction cannot be regarded as more than
tentative and promising.
CHAPTER IV
CANVAS
The usual, and probably the best fibre for the manu-
facture of canvas for painting is unbleached flax — that is,
linen; hemp and cotton are decidedly inferior. The
material is woven in different ways, and with strands of
different degrees of fineness, so as to produce cloths of
various degrees of thickness and fineness, and having
several kinds of texture and surface.
The canvas is first treated with size or a solution of
glue ; this should be as free from colour as possible : the
addition of honey to the size is undesirable. The priming
consists of two coats, the first containing whitening and
size, the second lead white and linseed oil. Fuller primings
are often given where it is not desired to allow the texture
of the canvas to remain evident. Such primings are put
on alternately in directions at right angles to one another,
and are treated in the same way as the primings of panel-
If before the last priming be dry it be dusted with zinc
white, or if a very thin final priming of zinc white and
drying oil (free from lead) be given, the usual discoloration
of the canvas which occurs on keeping it, especially in the
dark, will be avoided. But such discoloration can always
be removed by leaving in contact with the priming a
piece of blotting-paper saturated with a solution of
34
CANVAS
35
hydrogen peroxide : a slight warmth greatly hastens the
bleaching process.
Some painters in oil have employed with success a tem-
pera-priming on their canvases. This priming may be pre-
pared with a mixture of a strong, though elastic, size, with
whitening. A good composition of this sort may be made
by taking equal weights of fine whitening and of fine
plaster of Paris, which has been slaked in and soaked with
abundance of clean water, or of the preparation called
satin-finish, an artificial gypsum, used by paper-makers :
the warm size is incorporated with this mixture. When
the priming coats are dry the surface is dressed with a
layer of pure size, and allowed to harden thoroughly before
the picture is begun.
An ordinary primed canvas was examined with the
following results. The amount of moisture present was
5*5 per cent, of its weight, the priming 25 per cent., and
the dry substance of the size 15. The dry fibre which
constituted the remaining constituent would weigh, there-
fore, about 54 parts. It was further found, with the same
canvas, in a dry heat of 100° C. (212° F.) continued for
twenty minutes, that a strip 20 inches long became shorter
by a quarter of an inch, changing in colour from a creamy
white to a pale buff. After immersion in boiling water
for twenty minutes a piece of this canvas 20 inches square
was found to have shrunk more than i inch in one direc-
tion, and in the other direction rather more than half an
inch. The piece was somewhat crinkled, and had become
yellow in patches.
A few remarks as to the bearing of the above observa-
tions on some of the phenomena presented by oil-paintings
on canvas may be here introduced. The water present in
canvas varies with the temperature, and in consequence
36 PRESERVATION OF CANVAS
the dimensions of the canvas vary. As the contraction on
drying* and the expansion on taking up moisture are not
the same in the direction of the warp as in that of the woof,
there is an unequal strain upon the layers of paint upon the
surface. These may, therefore, become irregularly fissured,
and even loosened. The importance of selecting a canvas
so woven as to expand nearly equally in both directions is
evident, but the maintenance of a uniform temperature, and
of a suitable degree of moisture in the atmosphere where
pictures are hung, is also obvious. The absorption of
moisture by canvas occurs through the back, unless that
be also protected by paint. With the moisture deleterious
gases may also be absorbed, and these may easily pass
through and affect the priming, even the picture. Canvas
protected by panel behind, or coated at the back with a
layer of white lead which has been ground up with starch
paste, escapes this injury in great measure, as the sulphur-
etted hydrogen, etc., are then intercepted. The colouring-
matter of the fibre and size of the canvas may move towards
the front and discolour the priming and even the picture.
An excess of damp and a high temperature are the chief
causes of this movement. When the first priming coat con-
tains size, though it may adhere firmly to the sized canvas,
it may not hold the subsequent oil-painting quite so tena-
ciously. Canvas is liable to accidental injuries from
mechanical causes: a double canvas mitigates the evil. The
elasticity of the priming may not suffice, when the canvas is
rolled up, to prevent cracking. A small addition of a non-
drying oil, such as almond or olive oil, to the linseed oil
used in the priming coats, proves useful, but such addition
* Note that this contraction occurs at ordinary temperatures,
and must be distinguished from the contraction caused by boiling
water.
PRESERVATION OF CANVAS 37
should not exceed i part of non-drying oil to 20 of
drying oil.
There are two methods of preserving canvas from decay
by the application of solutions to the back after the priming
has been completed on the front. One of these solutions
contains corrosive sublimate (mercuric chloride) dissolved
in methylated spirit : a 5 per cent, solution is sufficiently
strong. The other solution is made by dissolving tannin
in methylated spirit. One or other of these solutions
should be applied once to the back of the canvas by means
of a broad stiff varnish-brush. When the spirit has
evaporated, the coating of white lead ground in starch
paste previously recommended in this chapter may be
applied. The two solutions we have named act by coagu-
lating some of the size in the canvas : the tannin turns it
into leather. Corrosive sublimate prevents the develop-
ment of mould or mildew, and is a good preservative
against the attack of animal organisms.
It may be observed that the employment of size in the
first preparation of canvas constitutes an element of weak-
ness. Many attempts have been made to substitute a less
hygroscopic and changeable substance. A plain collodion
containing a little ceresin (the hard paraffin from ozokerite,
or earth-wax) has been used for the purpose in question.
It is, however, very difficult to secure the adhesion to the
canvas of the film left behind when the collodion dries.
I have found that oil-paintings executed upon collodion-
ized canvas cannot be rolled up without damage. It
would probably be found that the formation of a viscose
film (see the account previously given of paper-sizing)
would prove a good substitute for ordinary size in the
first treatment of canvas.
^ The majority of pictures nowadays are painted on
38 PRESERVATION OF CANVAS
canvas, though it must be admitted that, especially in
respect of mechanical and chemical durability, it is a very
unsatisfactory material. Its light weight, its cheapness,
and the possibility of rolling up pictures painted upon it,
offer some advantages, no doubt, although the last charac-
teristic is of questionable value. On the whole, the draw-
backs to the use of canvas preponderate over its merits.
In this connexion special reference should be made to the
very marked alteration in dimension caused by damp (see
p. 36). This persistent working or movement of the ground
can be checked in the case of large collections gathered in
galleries duly equipped with the latest technical appliances
for the careful regulation of humidity and temperature.
But the case is different in private rooms where the move-
ment in question is practically unavoidable, and inevitably
leads to cracking in the course of time. This change is
still further promoted by the action of the oxygen in the
atmosphere, which attacks the painting both back and
front, and, by producing a gradual alteration in the binding
material, brings about a corresponding deterioration in
the adhesion and cohesion of the pigments. Moreover,
from the same cause, the linen or hemp fibre itself
becomes after a time so brittle that it is scarcely capable
of affording adequate support to the painted layer. When
it reaches such a state a picture must be ' relined ' — i.e.^
stretched on a fresh canvas. The painting itself may
thus prove more durable than the material base which
was designed to secure its durability.
^ The stability of pictures painted on canvas is en-
hanced if the back be protected by a metallic coating, so
as to protect it in a measure, on that side at least, from the
inroads of oxygen. The simplest way to secure this result
is by coating the back with tinfoil, using to fix the metal
PRESERVATION OF CANVAS 39
in position a strong solution of shellac* in spirits of wine.
In order to guard against the penetration of oxygen through
the accidental holes occurring in the tinfoil, a second
sheet of this metal may be added when the shellac solu-
tion has become dry. This treatment may be applied
generally to pictures painted on canvas, and virtually
doubles their span of life.
^ But protection from mechanical injury is not ensured
by a thin coating of tinfoil. If that further protection be
desired the back of the painting may be lined with sheet
metal. According to the size of the picture, sheet copper
or brass, or even sheet iron coated with tin or zinc, may
be employed. Paintings executed on canvas may be
fastened to the sheet of metal by the shellac "'^ cement
before named or with thick amber varnish. Thus the back
becomes protected both chemically and mechanically by
the same contrivance, and a high degree of durability is
thus assured for the picture.
H When it is a question of producing new pictures the
painting may be executed directly on metal. Such a
procedure applied to sheet copper was frequent with
Dutch miniaturists : the flawless condition of their works
justifies this method from the present point of view. Yet,
as this use of copper involves some risk that its green
and blue oxidation-products may give rise to discoloration,
a more appropriate painting-ground is offered by sheet
aluminium, which yields only colourless compounds.
Moreover, when aluminium is exposed to the atmosphere
there is formed on the surface a transparent and imper-
ceptible film of oxide which retains oil-colours very firmly.
Even on unprepared surfaces of aluminium it is possible
to paint very easily, for the metal possesses a peculiar
* Perhaps marouflage is preferable (see p. 26).
40 LINOLEUM
' tooth ' in relation to the paint so that one can readily lay
on successive coats of paint, stroke by stroke. Some
years ago I covered a piece of sheet aluminium with a
coating of oil-paint and exposed it in the laboratory to all
the accidents of the place. The paint remains sound to
this day and shows no tendency to crack or peel. A
sketch executed on strong sheet aluminium stands in a
greenhouse, where it receives all available sunshine and is
exposed to great variations of temperature : after exposure
during six months of spring and summer it showed no
signs of change for the worse. There is therefore good
reason for contending that in sheet aluminium we
possess an ideal painting -ground, especially for work
in oils.
H Another material which seems to lend itself particu-
larly to decorative and monumental painting is linoleum.
This consists of a very strong fabric coated with a thick
layer of oxidized linseed oil mixed with cork-raspings and
other materials. It thus bears some resemblance to canvas
which has been primed for the reception of oil colours, but
differs in its greater solidity and in the elastic substances
which it contains. The fact that the body of linoleum
consists mainly of the same substance that forms the bind-
ing material in ordinary oil-painting sufficiently guarantees
permanent union between picture and ground. From
another point of view the massive nature of the material
almost completely obviates the risk of mechanical injury,
and affords at the same time complete protection from the
attacks of air and damp at the back. If the brown colour
be not an objection, the surface can be used just as it is
as a painting-ground ; in this case the various rough and
smooth sorts of linoleum provide an agreeable choice of
surfaces from the artistic standpoint. But it will be
LINOLEUM 41
found better to lay on a thin coat of white oil-paint,
especially in the case of pictures intended to present a
bright general tone. Or a white pigment may be incor-
porated with the mass of the linoleum itself and so the
brilliancy of the applied colours may be enhanced. As
linoleum is manufactured several yards wide, it is possible
to paint very large pictures on a single piece, so that this
material seems more suitable for monumental or large
decorative works. It should not be fixed directly to the
wall, but attached to a metal framework covered with
galvanized iron wire netting and erected at a small dis-
tance from the wall ; the danger of injury from damp or
fracture is thus avoided.
With reference to the two materials, linoleum and sheet
aluminium, recommended in the preceding paragraphs for
use as painting-grounds, a few further observations may
be advisable. The variety of linoleum in which the
canvas-backing is omitted — solid linoleum — is not suit-
able for the purpose under discussion, for there is some
risk when large pieces are used of its sagging, and even
splitting, after the lapse of some time. And the experi-
ence of coach-builders is not altogether in favour of alu-
minium for the reception of a coating of oil-paint. In
some instances, at all events, there are signs of the dis-
integration of the metal and consequent cracking of the
superimposed layers of pigment. Possibly these draw-
backs may not occur where the painted surfaces are not
exposed to the external atmosphere. And it must be
remembered that the sheet aluminium of commerce varies
somewhat in its composition and properties.
' Willesden canvas ' appears to resist the attack of
moisture and of animal organisms: the copper-compounds
which it contains do not interfere with its use as a painting-
42 'WILLESDEN CANVAS'
ground, but the absence of soluble salts (chiefly sulphates)
from it should be ascertained. This test is easily made by
soaking a piece of the canvas weighing 50 grains in dis-
tilled water overnight, pouring off the clear liquid next
morning, heating it to boiling, and adding a few drops of
barium chloride solution and of dilute nitric acid. If a
distinct precipitate be formed the canvas cannot be em-
ployed safely, but a mere cloudiness may be disregarded.
It is just as well to test the watery extract for free acid by
means of litmus paper, because an attempt has been made
to remove the green colour of Willesden canvas by a bath
of dilute sulphuric acid, the presence of which is, on all
accounts, to be avoided. ' Willesden paper ' possesses
properties similar to those of Willesden canvas, and may
sometimes be found serviceable for work in oil-colours.
PART II
VEHICLES AND VARNISHES
Chapter V.— Oils. Chapter VI. — Resins, Waxes, Paraffin-Waxes.
Chapter VII.— Yolk and White of Egg, Size, Glue. Chapter VIII.
—Gums, Glycerin, Honey. Chapter IX.— Water- Glass, Lime-
and Baryta-Water. Chapter X. — Solvents and Diluents. Chap-
ter XI.— Siccatives and Dryers. Chapter XII. — Varnishes and
Oleo-Resinous Vehicles.
CHAPTER V
OILS
The common usage of the term ' oil ' is wider and less
definite than that sanctioned by chemists. We must
exclude from the category of true oils petroleum and
the liquid paraffins, spirit of turpentine and the volatile
essences of plants, the hydrocarbons of coal naphtha, as
well as a number of other liquids which present certain
superficial resemblances to the oils proper. Fats, how-
ever, belong to the same group, their solidity at ordinary
temperatures being, so to speak, an accidental rather than
an essential diiTerence.
The true oils are often called fixed oils, for they cannot
be boiled and distilled without change, thus differing from
really volatile liquids. They are glycevides — that is, com-
pounds from which glycerin, on the one hand, and fatty
acids, on the other, are obtainable. These glycerides are
named after the fatty acids which they yield. Thus olein
is the glyceride of oleic acid, linolein the glyceride of lino-
leic acid. In reality three kinds or varieties of glycerides
of each fatty acid are possible, but the oils used by painters
consist almost entirely of one of these kinds. The forma-
tion of one of these glycerides may be expressed in words
thus : One molecule of glycerin, reacting with three mole-
cules of a fatty acid, yields one molecule of the glyceride
45
46 DRYING OILS
in question and three molecules of water. Conversely,
under other conditions, one molecule of a glyceride, re-
acting with three molecules of water, produces one mole-
cule of glycerin and three molecules of fatty acid. If, in
this last reaction, we substitute for the water three mole-
cules of an alkali, such as potash, we obtain glycerin as
before ; but, in lieu of the free fatty acid, we find that an
alkaline salt of the fatty acid has been formed — such salt
is a soap. Alkaline soaps, namely, those of potash, soda,
ammonia, are soluble in water, which fatty acids — at any
rate, those with which we are here concerned — are not.
There are, however, other soaps which are insoluble in
water, namely, the lime, lead, copper, and many similar
metallic salts of fatty acids.
Oils, though insoluble in water, are easily soluble in
spirit of turpentine and other volatile plant essences ;
in benzene, chloroform, and liquid paraffins ; they are,
in fact, miscible in all proportions with these liquids.
There are other liquids in which the oils are less soluble,
such as alcohol, acetone, and glacial acetic acid.
Oils are divisible into two classes, one of which includes
those which dry up and harden, forming a kind of elastic
varnish, by exposure to the air. The oils of the other
class do not harden, but become sticky, and rancid in
smell ; these oils, however, if submitted to the tempera-
ture of boiling water for some time, do in some instances
become dry and hard, but the varnish they yield under
these circumstances is dark in colour and brittle ; it has
been suggested that some of these should be grouped to-
gether in a third class as * semi-drying' oils. The painter's
concern is almost exclusively confined to the oils of the
first group, generally known as drying oils. To the most
important of these attention will be directed presently,
LINSEED OIL 47
but the general methods of extracting them first demand
a few words of explanation. There are two different pro-
cesses in use. In one of these, which has been practised
widely from very early times, the oil is obtained by
pressure ; in the other process, invented some seventy
years ago, the oil is extracted by means of an appropriate
solvent. We may dismiss this latter process almost
summarily, for the product which it yields, though much
greater in quantity, is decidedly inferior to that obtained
by pressure. It is less fluid, and contains a larger propor-
tion of solid fats. The solvent commonly employed to
dissolve out the oil from oil-yielding materials is carbon
bisulphide (CSg), a compound of carbon and sulphur,
which may be prepared cheaply by passing the vapour of
sulphur through red-hot charcoal. Of the pressure-process
for obtaining fixed oils there are two modifications. In
the more usually adopted of these, the oily seed or other
material is first heated, and then pressed while still hot ;
in the other modification the pressure is applied to the
cold seed, etc. Heat and pressure give a more abundant
yield of oil, but the product is less pure and less well fitted
for use in painting. The bulk of the oils of commerce are
thus obtained. Cold-pressed oils remain clear in cold
weather, are more fluid than hot-pressed oils, and con-
tain a smaller proportion of solid fats and of free fatty
acids.
The most important drying oils are those of linseed,
poppy-seed, and walnut kernels; others are obtained
from niger-seed, sunflower-seed, and hemp-seed. The
first place is due to linseed oil.
Linseed oil is obtained from the seed of the common
cultivated flax (Linum iisitatissimimi). Linseed varies in
size and colour. The usual colours are a purplish-brown
48 LINSEED OIL
and a reddish- brown, but there is a nearly white sort — a
mere sport or variety — which may be said to be straw-
coloured. It is grown along with the brown variety in
some parts of the North- West Provinces of India, par
ticularly in Nagpur, but no pains are taken to keep the
strain pure. Through the kind offices of the Director of
the Royal Gardens, Kew, the Government of India were
good enough to obtain a specially pure sample of some
hundredweights of white Nagpur linseed, and to place it
at my disposal. Attempts to grow it for seed in this
country and in Belgium failed, but a large quantity of
oil was expressed for trial and analysis. Messrs. Bell
and Co., of 225, Oxford Street, obtained several gallons
of oil by cold-pressure ; many artists have expressed
their approval of the product. One advantage of this
white seed is the ease with which the purity of a sample
may be recognised by the eye, any accompanying weed-
seeds differing widely in colour from the white linseed.
The skin of the seed is, moreover, thin, the cold-drawn
oil is nearly colourless, and the seed is particularly rich
in oil, containing no less than 45 per cent, of its weight,
although, of course, much less than this proportion is
obtainable by cold-pressure. In a hand-press about
25 per cent, was the average yield. Of the common or
brown linseed our chief supplies come from Russia and
India. The Russian seed is generally finer than the
East Indian ; it is, moreover, imported in a less mixed
and impure condition. By screening, the greater part of
the impurities are or may be removed, but it is sold on a
basis of 4 per cent, impurity. The impurities consist of
dirt, other oil-seeds, such as mustard, rape, and gold of
pleasure, and non-oily weed-seeds. The presence of the
last-named, though it reduces the yield, is not otherwise
LINSEED OIL 49
objectionable,* but the same remark does not apply to
the foreign oil-seeds. Most of these contain non-drying
oils, which mingle with the linseed oil when the sample
is pressed and reduce its siccative character. Much
linseed now comes from the Argentine, Canada, and the
United States, as well as from India and Russia.
The percentage of oil in linseed varies between 28 and
45 : by cold-pressure 20 per cent, is the average yield ; by
hot-pressure, 27 per cent. ; by extraction with carbon
disulphide, 33 per cent The linseed oil in common use
by artists is hot-pressed oil, and is very rarely, if ever,
obtained from absolutely pure seed. The seed should be
kept three months before it is pressed. The expressed
oil should be exposed to light in covered glass vessels or
tanks, and kept at a temperature of 212** F. for some
time. It thus loses colour and becomes clear, a slimy
deposit containing mucilage, albuminoid matter, and
traces of a cyanogenetic glucoside, being formed. When
thus bleached and clarified, the oil should be preserved
in corked bottles filled quite full ; the longer it is kept, the
better it becomes for painting, provided the access of air
is prevented. The specific gravity of good linseed oil
varies very little. At 60° F. (15-6° C.) it is -935 ; a bottle
which will hold 1,000 grains of water at this temperature
will therefore hold but 935 grains of hnseed oil. It
expands considerably with heat, its specific gravity at
50° C. being -913 only. One part of linseed oil requires
36 parts of cold absolute alcohol for solution, but only
4 parts of boiling alcohol. It may be purified by solution
in boiling alcohol or in petroleum ether. Other methods of
purification are generally employed. Amongst these may
* Occasionally these weed-seeds give up, under pressure, certain
matters which deepen the colour of the expressed oil somewhat.
4
50 LINSEED OIL
be named the following : Filtration through felt or carded
cotton and charcoal, and then through pyrolusite ; con-
tact for some weeks with 3 per cent, of a mixture of
equal parts of kaolin and aluminium hydrate, both these
compounds having been previously dried at about 50° C;
agitation with a solution of common salt, followed by
washing wth water, and drying by a heat of 220° F. ;
treatment with one four-hundredth part of oil of vitriol,
addition of hot water, washing, and drying. Various
other processes and reagents have been employed for
purifying and bleaching linseed oil. Aqueous solutions
of sulphurous acid, green vitriol, potassium permangan-
ate, potassium bichromate, and peroxide of hydrogen
may be included in this list. The addition of i per cent.
of oil of turpentine to the oil, and then passing a mixture
of air and steam through it, has also been tried. What-
ever process be adopted, no acid, saline matter, or
moisture must be left in the oil. The general and usual
result of all the very different kinds of treatment to
which linseed oil is subjected, in the above-named and in
many other processes, seems to be the more or less com-
plete removal of impurities. The effect on the properties
of the purified oil is chiefly seen in its greatly increased
rate of absorbing oxygen and consequent hardening.
The chemical composition of linseed oil may now
engage our attention. Its ultimate analysis shows it to
vary according to the method of extraction adopted, cold-
pressed oil containing about 78 per cent, of carbon, 1 1 per
cent, of hydrogen, and 1 1 per cent, of oxygen ; while the
hot-pressed oil contains nearly 3 per cent, less carbon,
and nearly 3 per cent, more oxygen — linseed oil, extracted
by carbon disulphide, is still poorer in carbon, and richer
in oxygen. It appears that linseed oil consists chiefly of
LINSEED OIL 51
three glycerides, called, respectively, linolein, linolenin,
and olein. A small, but variable, amount of free fatty
acids, such as palmitic and arachidic, is also present.
The empirical formulae of the three fatty acids of the
above-named glycerides are, respectively :
Linolenic Acid - - CJ8H3QO2.
Linoleic - - - - CigHggOa.
Oleic - - - - CigHgjOg.
Linolein, which is present in linseed oil to the extent of
about 20 per cent., is the glyceride of linoleic acid, and
has the formula (C^^2.fi)^,C^ll^jO^ ; or, as it may be
written, 03115(0, C^gHg^Oyg. The relation of this glycer-
ide to glycerin may be seen when the latter body is ex-
pressed by the formula, C3H5(OH)3. It is probable that
the other main constituent of the oil — linolenin — is a
similarly constituted glyceride, and that it closely resem-
bles linolein in physical and chemical properties. When
100 parts of linseed oil are saponified by an alkali, they
yield from 9-4 to 10 parts of glycerin.
The most important chemical property of linseed oil,
from a painter's standpoint, is its behaviour with oxygen.
Under certain circumstances, it absorbs oxygen to the
extent of 13 or even 14 per cent, of its weight, becoming
converted into a mixture of substances for which it is con-
venient to retain the old name linoxine. Linoxine is solid,
and not liquid ; it is far less soluble than linseed oil in any
solvent, and in many liquids it is insoluble. Linoxine
is, moreover, denser than the original oil ; 100 grains of
linseed oil produce about 109 or no grains of linoxine.
Notwithstanding the greater density of linoxine, when
compared with the original oil, its formation is attended by
a considerable expansion. In consequence, a layer of raw
52 LINSEED OIL
linseed oil spread upon glass becomes wrinkled during the
drying and oxidizing process. During the oxidation of
linseed oil, the small quantity of olein it contains remains
unoxidized — its presence confers elasticity upon the pro-
duct.
H The incidents associated with the hardening or solidi-
fying of drying oils have always been an attractive study,
but it is only through the chemico-physical researches of
recent years that their true nature has been made clear.
The most important points will now be briefly explained.
This hardening depends, as before stated, on a process of
oxidation — that is, on the absorption of free oxygen from
the atmosphere. During this process carbon dioxide and
other volatile organic compounds are formed and given
off, while simultaneously there are produced solid, non-
volatile bodies which constitute the dried and hardened
oil. These solids then, in their turn, by a further and
very slow oxidation, yield other volatile products. While
in the first stage, the gain in weight of the oil, due to the
absorption of oxygen, far more than compensates for the
loss which arises from the escape of volatile matters, in
the second stage there is a distinct diminution in bulk and
in weight, while the residue acquires a deepening brown
hue. This last phenomenon, however, does not seem to
be inevitable, for it occurs when light is excluded ; and
this yellowing or embrowning of the hardened oil may
often be remedied by subsequent exposure to light,
although it does occur in many pictures which hang on
well-lighted walls. As this change does not take place
always, it seems reasonable to conclude that, putting
aside discoloration through the deposition of dirt and
sooty matters, it must arise by an alteration in something
present which is not oil. Indeed, it seems to be trace-
LINSEED OIL 53
able to the presence of lead introduced into the oil used
as a siccative or dryer. As equally efficient dryers may
be prepared without the employment of lead compounds,
it seems desirable that these should be rejected.
U If a thin layer of linseed oil be put into a bottle full
of air and provided with a contrivance for observing the
absorption of the oxygen, it will be seen that at first the
oxidation proceeds very slowly — this is Period I. The
absorption then becomes more and more rapid till it
reaches a maximum — this constitutes Period 11. Once
more the absorption becomes slow — this is Period III.
It is only during this last period that the oil loses its
fluidity, becoming first viscous and finally solid. Oil
examined at the close of the second period or stage dries
quickly, and is often called varnish.
%_ The processes just described, if carried out in the
dark, occupy some weeks, but they may be hastened by
means of several different agents. For instance, by heat —
that is, by raising the temperature. It is a general law that
the rate of chemical action increases proportionately with
rise in temperature. Thus the higher the temperature at
which the linseed oil absorbs oxygen, the more ra,pid
does the action become. It is on this fact that the very
old process of varnish-making by boiling raw linseed oil
rests. The oil, heated in open vessels up to a fairly high
temperature (200° C. or more) absorbs oxygen rapidly,
while at the same time, principally through the overheating
of the sides of the vessels used, some brown decomposition
products, which give the varnish a dark colour, are formed.
If the heating be stopped when the oil is in its second
stage of oxidation, a product is obtained which, though
still fluid, has acquired the property of absorbing with
great rapidity the small amount of oxygen still needed to
54 LINSEED OIL
render it solid. Oil thus treated dries, therefore, much
more quickly than raw oil, and is used when quick drying
is of importance ; it is known as linseed oil varnish.
However, along with the advantage of quick drying, its
dark colour constitutes a drawback. But this discolora-
tion, being due to excessive and unequal heating, can be
avoided by conducting the operation of limited oxidizing
at a moderately high temperature (say ioo° to 150° C).
It may, too, be hastened by passing a stream of air
through the heated oil. In this way, not only is discolora-
tion avoided, but the oil is actually bleached, and a nearly
colourless varnish produced. If the treatment be stopped
at the right moment, a sufficiently fluid varnish is ob-
tained, while if the process is carried farther, viscous and
almost solid products are formed. Naturally this mode
of preparing the varnish takes more time, owing to the
lower temperature employed, than the old boiling process,
but the product is much better.
IT The second agent which may be employed to hasten
the oxidation of oil is light, which acts energetically. On
this fact depends the process of transforming linseed oil
into varnish by exposing it to sunshine in shallow vessels,
so as to facilitate the access of light, care being taken to
exclude dust, while the oil is occasionally stirred in order
to prevent a skin being formed on the surface through a
superficial oxidation. If this skin be produced, it hinders
the access of oxygen to the oil beneath, while the product
is not homogeneous. It is on this action of light that a
common practice of artists is based, the placing a picture
in the sun that it may dry quickly. The converse prac-
tice, however, of keeping paintings soft and moist during
the time when the artist is not working upon them, by
excluding light from them, is less known. The best
LINSEED OIL 55
means of securing this object consists in arranging a
sheet of aluminium in front of the picture so as not to
touch the surface ; it is light in weight, and rigid.
U A third agency which may be used to hasten the oxi-
dation of a drying oil is to be found in the presence of one
or other of a group of substances, the mere presence of
which when the oil is still in its first stage may reduce the
time required to reach a condition of solidity from days
to hours. Some compounds of lead and manganese are
the best known of these active materials. Innumerable
old recipes for varnish boiling and for preparing varnish
in the cold are based on the solution in oil, in one way or
another, of certain compounds of the metals just named.
Since it is only within comparatively recent times that the
conditions, under which the oxidation of drying oils pro-
ceeds, have been clearly understood, it is to be expected that
these old recipes should prescribe a number of superfluous
and, indeed, actually injurious operations. In order to
obtain a quickly drying oil it is requisite merely to dissolve
a suitable and generally a very small quantity of one of
these ' accelerators ' in the oil, and to carry the oxidation
process no farther than to the second stage or period.
Lead, which has been longest known as a ' dryer,' is best
used in the form of its linoleate or resinate, as these com-
pounds dissolve in oil, especially when warmed. Strong
solutions of this kind, generally dark in colour, are put on
the market as siccatives, and added by artists to their
oil-paints on the palette in a haphazard manner. Such is
an injudicious procedure and may have an injurious effect
on the durability of the picture. For when thus added
the dryer is more or less unevenly distributed, not only
throughout the picture as a whole, but even in different
portions of the same paint. Thus there ensues uneven
56 LINSEED OIL
drying and the formation of cracks is promoted. It is
better to add the proper quantity of the dryer to the
paints in the first instance.
During recent years many improved methods of treating
raw linseed oil have been devised. The ' boiling ' has
been carried out in aluminium vessels in lieu of those of
iron, which become much corroded through the action of
the free acids of the oil upon this metal. The iron com-
pounds thus formed are undesirable ingredients of the
boiled oil. Then, again, superheated steam has taken
the place of direct fire-heat in treating the oil, which is
mechanically stirred while a current of oxygen or of air
is at the same time sent through the liquid. In yet an
other process, the raw oil in the form of spray and heated
is brought into intimate contact with a stream of heated
air. There exists also an improved method of preparing
a drying oil by the agency of light. As it has been found
that sunlight acts mainly in virtue of its ultra-violet or
actinic rays, an artificial light, singularly rich in such
rays, has been substituted for the light of the sun. This
is found in the mercury lamp, in which an electric spark,
passing between two mercury poles enclosed in a quartz-
glass tube, originates a peculiar light of high activity. A
battery of such lamps is so arranged that their radiation
impinges upon the oil, which is at the same time kept at
a temperature of 80° C, while a finely divided stream of
oxygen gas is forced through it. The pale, quickly drying
oil thus produced is said to possess the property of drying
uniformly throughout without the production of a skin on
the surface.
The changes which occur during the oxidation of
linseed oil, as described in the preceding paragraphs,
are, as will have been seen, complex ; but there is
some formic acid formed, so that the product is sour —
LINSEED OIL
57
carbonic acid gas and water are also produced. It
has been shown that there are many ways of bringing
about this oxidation. A very common one is to heat
the oil to a temperature of at least ioo° C, and to blow
air through it, or air containing ozone. Many sub-
stances favour the absorption of oxygen by linseed oil
under the above conditions. Amongst these may be
named manganese dioxide, borate, oxalate, resinate, or
linoleate ; cobalt resinate ; red-lead, litharge, or lead ace-
tate ; green vitriol, iron in the presence of water, etc. It
is better to use one of the manganese compounds, and an
excellent result is obtained with the borate of this metal.
On the small scale, the operation may be thus carried out :
Tie up in a small piece of muslin 20 grains of dry and
powdered manganese borate. Suspend the bag in a glass
quart flask, into which a pint of linseed oil has been
placed, so that the bag is just covered by the oil ; lightly
plug the mouth of the flask with some carded cotton.
Stand the flask in a warm place, where the temperature
does not fall below 40° C, nor rise above 100° C. In a
fortnight's time, the oil will have become strongly sicca-
tive, so that when it is spread in a thin layer on glass, or
paper, it will dry up to a tough varnish within twenty-
four hours. If the oil and manganese borate be main-
tained, by means of a water-bath, at a temperature of
100° C, the change will occupy less time, and the product
will be just as good ; but it is not advisable to hoil the oil
with the borate, although the change may be thus eft'^ected
in less than an hour. The oxidation may be further
hastened by occasionally blowing a little air into the oil
through a glass tube kept permanently in the flask. When
the rapid-drying quality of the oil has been proved, by
experiments made with a drop or two Vv^ithdrawn for that
purpose, the flask is allowed to get cold, and the oil
58 SICCATIVE LINSEED OIL
poured into a corked glass bottle, so as to fill it. In the
course of the next few weeks, a slight deposit will be
formed in the bottle ; when this has occurred, the clear oil
should be poured off into other bottles, and preserved for
use. According to the purpose for which the prepared oil
is to be afterwards used, the treatment with the borate must
be more or less prolonged ; but care should be taken not to
carry it so far that the oil becomes ropy or viscous, unless
it is intended to make linseed oil varnish. In the subse-
quent chapters of this book we shall often refer to this
siccative linseed oil as ' manganese oil.' To the above
directions for preparing this oil may be added the remark
that if the operations be conducted in a strong light, the
oil will be bleached, as well as rendered highly siccative.
No satisfactory explanation of the action of the manganese
borate (and of many other substances used for the same
purpose) has been offered. But it seems probable that
the absorption of oxygen by the oil is favoured by the
removal of certain impurities, and this the borate of
manganese may effect : it has been suggested that the
action is in part catalytic.
The increasing specific gravity of the ' manganese oil,'
as the process is prolonged, may be used as an indication
of the point at which the heating may be discontinued.
When the oil has acquired a specific gravity of -945, it is
generally sufficiently siccative for grinding with non-
drying pigments, and as an addition to certain varnishes.
For these purposes it may even attain a specific gravity
of -96 ; but when it shows '99, or '995, it constitutes a
thick varnish, which needs dilution with a suitable
solvent. It may be well to remark here that the various
processes for rendering linseed oil more rapidly- drying
may be regarded as resulting in two actions, partly con-
TESTING LINSEED OIL 59
secutive, partly simultaneous. The first action, if it
could, or did, occur alone, would yield a purified oil apt
to dry quickly, but very slightly altered in composition ;
the second action is more profound, and gives rise to a
thickened, denser product, in which the drying process
has already commenced. In practice, the first action
occurs almost, but not quite, uncomplicated with the
second, when linseed oil is warmed with borate of man-
ganese in a vessel to which atmospheric air has very
limited access ; the second action, which is of necessity
associated with the first, takes place when a stream of
air is blown through warm linseed oil, even in the absence
of manganese borate, but far more quickly in its presence.
The superiority of the highly siccative oils prepared
with borate of manganese (or the oxalate, resinate, or
linoleate) over those in the manufacture of which lead
compounds are used, is so decided that all description of
the older and less satisfactory methods will be omitted.
But there are two other ways of rendering linseed oil
more siccative, which deserve a passing notice. Into a
clear-glass quart-bottle an ounce of distilled water and
an ounce of clean iron brads are first placed, and then
one pint of raw linseed oil, agitation being avoided. The
next day, the bottle, placed in as strong a light as possible,
is to be shaken frequently, the shaking being repeated
every day, until a drop of the oil, when tested, shows a
sufficient degree of drying character. Finally, the liquid
part of the mixture in the bottle is poured into a separating-
funnel, and the aqueous part allowed to run away. The
oil may require drying and filtration. In another similar
process green vitriol is substituted for the metallic iron,
the other directions being identical.
The most important property of linseed oil, and some
6o TESTING LINSEED OIL
methods for the further development of this property
having been discussed, we may now describe the remain-
ing characters of this oil. The cold-pressed oil is very
pale straw-coloured, or pale yellow, with occasionally a
faint greenish hue ; the hot-pressed oil is a darker yellow
or brown. The cold-pressed oil, when considerably
cooled, remains clear long after the hot-pressed oil has
become turbid. The fluidity of the oil is less than that
of water in the ratio of i : lo. The hot-pressed oil has a
much stronger taste and odour than the cold-pressed oil.
The adulteration of linseed oil with other oils may be
recognised with more or less precision by means of
several different tests. Most of these tests (oil of vitriol
test, nitric acid test, etc.) produce reactions in which the
oil and the acid acquire varied colours characteristic of
different oils. These tests must be applied under exactly
similar conditions of temperature, agitation, lapse of time,
strength of acid, etc. ; and even then, unless the experi-
menter is well-versed in the work, the indications obtained
are sometimes perplexing and difficult to interpret. The
amount of iodine absorbed by a given weight of linseed
oil is also a measure of its drying power as shown in its
capacity for absorbing oxygen. This 'iodine-value,' as
it is called, is the amount of iodine absorbed from
chloride of iodine in the presence of glacial acetic acid,
by loo grammes of oil. The iodine-value for linseed oil
is somewhere near 200; the figures for walnut oil and
poppy oil are always lower, while the semi-drying and
the non-drying oils may not show half this value, and
these are the oils likely to be used as adulterants. But
such quantitative determinations can be properly per-
formed only_ by the skilled chemist. There is another
test known as Valenta's acetic acid test, which has been
POPPY OIL 6i
used for the detection of non-drying oils in linseed oil.
It is based upon the less solubility of the former oils in
glacial acetic acid. It is, however, not easy to secure
constant results with this test. Determinations of
volatile matters, of unsaponifiable substances, and of
insoluble bromine derivatives, afford valuable indications
as to the purity of samples of this oil.
The specific gravity of linseed oil also affords a valuable
means of testing its purity. At 15*6° C. (60° F.) it is
denser than most other vegetable oils :
Name of Oil Spec. Grav.
Name of Oil Spec. Gr
Linseed - - -935
Poppy-seed - - "926
Gold-of- Pleasure- -931
Sunflower-seed - '925
Hemp-seed - - -930
♦Black Mustard-seed- -921
Cotton-seed- - -930
♦Ground-nut - - "918
Walnut - - -927
♦Colza-seed - - -914
The four oils marked with an asterisk are practically non-drying.
Poppy Oil. — This oil is obtained from the seed of the
opium-poppy, Papaver somnifenmi. It is of a very pale
straw-colour, often almost colourless, and is nearly free
from taste and smell. By filtration through hot animal
charcoal it may be completely decolourized. If the fluidity
of water be represented by 1,000, that of poppy oil at
15-6° C. is 74. Its specific gravity at the same tempera-
ture is -926. Its chemical composition is near that of lin-
seed oil ; it contains the same three glycerides, but in
different proportions, for it is mainly made up of linolein
and olein. The large quantity present of olein causes
poppy oil to be a less rapidly drying oil than linseed.
Wolffen, in 1640, stated that poppy oil dries thvoughout in
four or five days, while linseed oil forms a pellicle upon the
surface. Joseph Petitot, writing from Geneva under date
January 14, 1644, stated that umber is a siccative for
62 NUT OIL
poppy oil. Poppy oil was introduced into painting in the
beginning of the seventeenth century, after linseed and nut
oil. Later on in the same century the Dutch painters ac-
quired greater confidence in this more slowly drying oil,
employing it not only in the painting process, but also for
grinding their pigments, especially whites, blues, and pale
tints.
Nut Oil. — This oil is obtained from the kernels of the
common walnut, Juglans vegia. Leonardo da Vinci directs
it to be made from the peeled kernels in order to avoid the
chance of darkening its colour, and also causing the subse-
quent alteration of the tone of the pictures painted with it.
The kernels were to be soaked in water first, before being
peeled and pressed. The introduction of nut oil into
painting followed that of linseed oil, and preceded that of
poppy. Cold-pressed nut oil is much paler in colour, and
has much less taste and smell than the hot-pressed oil ; it
also differs in composition much in the same way that cold-
pressed differs from hot-pressed linseed oil. The con-
stituent glycerides of nut oil are the same in kind as those
of linseed oil, but a larger proportion of linolein is present.
Nut oil closely resembles linseed oil in its physical charac-
ters ; its specific gravity, '929, is intermediate between
that of linseed and poppy oil : cold pressed oil from the
Black walnut {Juglans nigra) has the specific gravity '922,
and is quite as good for painting purposes as oil from the
common walnut. Besides the three drying oils already
described we may name that expressed from niger-seed,
Guizotea oleifera. It is occasionally employed in grinding
artists' colours as a substitute for linseed and poppy oil.
Tea-seed and camellia-seed oils, and the oils extracted in
Japan from the seeds of Perilla ocimoides and from the
kernels of Torreya nucifera^ are not of sufficient importance
ACTION OF PIGMENTS ON OILS 63
to demand description. There is, however, one remark-
able drying oil of recent introduction which ought to be
named here. This is Chinese Wood oil or Tung oil ob-
tained from the seeds of A leurites cordata. It is distinguished
from the oils we have been discussing by the change
which it undergoes when heated to about 282° to 285° C.
After having been maintained at this temperature for a
few minutes the oil becomes a gelatinous mass, firm and
free from stickiness. Tung oil is heavier than any of the
oils hitherto described. It is probable that it may find
certain applications in artistic painting, for it has been
shown to yield a durable film when oxidized.
A few observations may now be offered as to (i) the
action of certain pigments on oils; (2) the different amounts
of oil needed for grinding with different pigments.
I. Action of Pigments on Oils. — The most common action
is a physical one, in which the opacity of a pigment is
gradually lessened in course of time by the more complete
interpenetration of the oil between the particles. Thus
yellow ochre and raw sienna, for example, darken in colour
because they become more translucent, just as a piece of
oiled cream-laid paper is darker and yellower than the
same paper when dry. The light which falls upon it
plunges into it more deeply, and on reflection is more
highly coloured. In the case of such pigments as we
have named, and several others, another cause is at work
darkening and modifying the colour : this is the yellowing
of the oil itself. And it is the pigments which require the
largest proportion of oil for grinding which exhibit in a
marked degree the phenomena in question.
A second action between a pigment and the oil with
which it has been ground is the peculiar gelatinous or
* livery ' condition quickly assumed by some oil-paints.
64 OIL IN PAINTS
This change is particularly noticeable with the cochineal
and madder lakes. I have succeeded in obviating it, by
carefully drying the pigments at a temperature just under
ioo° C, before grinding them with oil, and by substitu-
ting for raw linseed oil a mixture of the ' manganese oil,'
described in the present chapter, with some poppy oil.
Those pigments which dry easily should be ground with
more of the latter oil, those which dry with difficulty with
more of the former. Sometimes pigments harden quickly
in the tube itself ; this change is due either to the siccative
character of the pigments, or to the introduction of an
actual * dryer,' or to the too copious use of a strongly sic-
cative oil with those pigments which are naturally slow
in drying.
The third action between a pigment and the oil with
which it has been ground appears to be of a distinctively
chemical nature. The most striking example of it known
occurs with flake-white. When normal flake-white, that
is hydrato-carbonate of lead, is ground in oil and after-
wards exposed to the air, it gradually becomes very
hard — much harder than the great majority of other oil-
paints under the same conditions, zinc-white for example.
It is not the hardness of the lead-compound plus the
hardness of the oxidized oil, but a hardness combined
with toughness of a higher order. As those varieties of
white-lead which consist wholly of the carbonate do not
possess this quality in anything like the same degree, so
it must depend upon the lead hydrate which is intimately
associated with the carbonate in typical white-lead. It
has been usual to conclude that the phenomenon is due
to the formation of a lead soap, a linoleate of lead, by the
interaction of the free acids in the oil with some of the
lead hydrate. This view is supported by some, yet hardly
ACTION OF PIGMENTS ON OILS 65
decisive, experimental evidence. Thus the longer the oil
and the lead-white remain in contact the more marked is
the change, especially if the temperature be raised some-
what above the normal. If a collapsible tube, filled with
ordinary flake-white ground in oil, be tightly closed so as
to exclude the air and then be submersed, in water kept
hot, for a few days, the change in question is hastened.
Consequently it becomes impossible to extract from the
paint so treated, by means of ether or other suitable
solvent, quite the amount of oil originally present. One
could understand this result had the paint been allowed
to absorb oxygen from the air, when some linoxine, in-
soluble in most solvents of oil, would have been pro-
duced. Whatever the action may be, and whatever the
true explanation, the phenomenon is beyond question
Further reference to this subject will be found in Chapters
XIII. and XXIII.
A fourth action between a pigment and the oil with
which it has been ground is occasionally observed with
certain colours of organic origin, which actually dissolve
in and stain the oil. Bitumen, gamboge, and several
alizarin preparations exhibit this phenomenon.
2. The different amounts of oil required by different pig-
ments may now be considered. As a rule, the densest or
heaviest pigments require the least oil. A few pigments
require an excess of oil in order to protect them from
moisture or other injurious agents. Different authorities
do not agree at all closely as to the amount of oil needed
to make a workable oil-paint from the same pigment.
The following list gives the weight required by 100 parts
in weight of 22 pigments :
66
OIL IN PAINTS
According to
According to
Name of Pigment
C.
Roberson and Co.
Winsor and
(1901)
Newton (1901)
White Lead -
- 16 -
- 15
Zinc White -
- 19 -
- 23
Aureolin -
- 71 -
- 49
Chrome Yellow
- 35 -
- - 56
Cadmium Yellow
- 37 -
- - 67
Yellow Ochre -
- 59 -
- - 63
Raw Sienna -
- 147 -
- 240
Vermilion
. _ -
- 23
Light Red
. 69 - -
- 70
Madder Lake -
- 103* - -
- 55
Terre Verte -
- 49 -
- - 87
Viridian -
- 56 - -
- 52
Prussian Blue
- 72 -
- - 78
Cobalt Blue -
- 50 -
- 90
Ultramarine (artificial) - 34 -
- 43
Raw Umber -
- 97 -
- 95
Burnt Umber
- 97 -
- - 87
Bitumen -
. _ .
- 127
Brown Madder
- 81 -
- 93
Vandyke Brown
- 72 -
- 94
Burnt Sienna
- 138 - -
- 150
Ivory Black -
- 88 -
- 112
The discrepancies between the corresponding figures in
the vertical columns are due, amongst other causes, to
differences in the modes of preparation of the dry pig-
ments ; to natural variations in the native earths em-
ployed ; to the dissimilar standards of solidity or fluidity
aimed at in the finished paint ; and to several other causes
which it is needless to particularize, but amongst which
may be named different modes of grinding and the em-
ployment of different kinds of oil.
The great differences in the above amounts of oil do not
cause such serious results in the conduct of the process of
Figure for rose madder.
OIL IN PAINTS 67
oil-painting as might have been expected at first, for they
correspond in a measure to the relative bulks of the
several pigments. We can use more copal or amber
varnish to balance the excess of oil in some pigments, and
so secure a uniformity of structure, texture, and rate of
drying in the different parts of the work. It is, however,
often convenient to remove some of the excess of oil from
a pigment before using it, especially with the colours pre-
pared by some makers.* This can be done by leaving
the oil-paint on a pad of blotting-paper; but 3-inch cubes
of plaster-of-Paris afford a far cleaner and surer method
for the absorption of oil. It may be further remarked that
the quantities of oil required by some of the pigments in
the above table may be reduced by grinding them under
greater pressure. Raw sienna, burnt sienna, and ivory
black should be dried at 100° C. just before grinding, and
then yield workable paints with less oil. The subsidence
of vermilion from the oil in which it has been ground
may in some measure be prevented by using ' manganese
oil ' instead of raw linseed oil, and adding to it a small
quantity of linoleate or oleate of alumina, or of beeswax,
or of hard paraffin wax or ceresin, having a melting point
not under 65° C. Some artists find it a good plan to keep
their tubes of vermilion and of other heavy pigments in
an inverted position — that is, with the cap downwards.
* Dr. H. Stockmeier, of Niirnberg, found the following per-
centages of oil in certain oil-paints from different sources which he
analysed :
Flake- White (Robersonand
Co.) ... - i6-2
Light Red (Winsor and
Newton) - - - 41 '9
Burnt Sienna (Dr. Schoen-
feld) - - - - 59 '2
Chinese Ochre (G. B.
Moeves) - - - 45
CHAPTER VI
RESINS, WAXES, AND SOLID PARAFFINS
In commercial parlance resins are incorrectly termed gums.
The true gums (Chapter VIII.) are either soluble in water
or swell up in that liquid, but resins are not acted on by
water. The term resin is used throughout the present
volume in its proper sense, so that ' copal resin,' * mastic
resin ' are spoken of, not ' gum copal,' ' gum mastic' All
the resins used for making vehicles and varnishes are of
vegetable origin; they contain besides carbon and hydro-
gen a not inconsiderable proportion of oxygen. They are
related to the hydrocarbons known as terpenes, present in
many essential oils, but are of more complex constitution.
Some resins, such as gamboge, contain gum and are called
gum-resins ; others contain a hydrocarbon (or terpene, see
Chapter XI.) or an aromatic acid, and are called balsams ;
others are true resins, but even these rarely, if ever, con-
sist of a single definite compound, but are mixtures of at
least two, often of three, four, or five different bodies.
Generally these constituents of true resins differ as to their
degree of solubility in various liquids, such as alcohol,
ether, spirit of turpentine, benzene, petroleum spirit, and
heated fixed oils. They contain carbon, hydrogen, and
oxygen, with occasionally a little sulphur, and are usually
of an acid character, and are capable of forming soaps,
68
AMBER 69
called resinates, with the alkalies. Resins differ much
from one another, not only in solubility but also in hard-
ness and in the temperature at which they melt. Those
which are least soluble are generally those which are
hardest, and which require the highest degree of heat to
bring them into fusion. Most true resins contain, besides
their proper resinous constituents, small quantities of
colouring-matter, of water, of crystalline aromatic acids,
and of a volatile hydrocarbon or terpene. All these im-
purities, save the first, may be removed, generally with
advantage, by the following treatment. The powdered
resin is thoroughly mixed with a little water and placed
in a large glass retort. A current of steam is then passed
into the mixture until the terpene and volatile acids
present have distilled over. To the contents of the retort
carbonate of soda is added (i part for each 100 of resin).
The mixture after agitation is allowed to cool and then
filtered through a fine cotton cloth. The purified resin is
then washed on the filter with distilled water, then dried
in the air and finally in the water-oven : the air-bath and
a temperature of 110° to 120° C. may be used for the
desiccation of the harder resins.
It might be thought that the subject of resins would be
sufficiently discussed from the painter's standpoint by a
description of three kinds — amber, copal, mastic. But it
will be shown presently that copal and mastic are names
given to several distinct substances, and that there are some
other resins which cannot be excluded from our view.
Amber is the most familiarly known of all the resins on
account of its long use in its natural state for ornamental
purposes. Amber beads have not infrequently been found
in early British graves ; on the Continent these and other
ornaments of amber have often been obtained from ancient
70 AMBER
interments. At Naples I was shown some years ago a very
large number of antique fibulae carved out of this sub-
stance : they had just been disinterred from Etruscan
tombs. Such amber has often become brittle and far more
soluble in the usual solvents, especially so far as regards
the surface layers ; but in other instances the preservation
of the properties of this resin has been complete. The
chief localities where amber is found are the Prussian
shores of the Baltic Sea (particularly between Konigsberg
and Memel) and the neighbouring plains; it has been
found in veins, and is regularly quarried. Some amber,
much of it having a dark colour, is found near Catania,
Sicily. Near Lemberg (Galicia in Austria) nodules of
amber occur in rock. It occurs in several places in Den-
mark, Sweden, Norway, and France. In the British
Museum collection of minerals there is a fine mass from
Cambridge. Excellent specimens occur in comparative
abundance on the seashore at Southwold in Suffolk, and
at several other places on the Suffolk, Norfolk, and Essex
coasts. The dark fossil resin found in Birma, often in
large masses, is not identical with Baltic and English
amber. The same observation may be made with respect
to the so-called ambers of Travancore in the East Indies,
and of the Isle of St. Louis, Senegambia, Africa. In
fact, amber, instead of being, as commonly stated, the
fossil resin of a single species of tree of Tertiary age, has
obviously been derived from no inconsiderable number of
different plants. Goppert, so long ago as 1853, satisfied
himself that at least eight species of plants besides Pinites
siiccinifev have afforded this fossilized resin : he also
enumerated 163 species of plants as represented by remains
in amber ; many others have been since recognised.
Amber has a specific gravity of about 1*07 ; its hard-
AMBER 71
ness is 2J on the ordinary mineralogical scale. In most
of the usual solvents of resins it is either insoluble or but
partially soluble. When heated quickly on a spatula it
splits up and then fuses into a viscous liquid, the drops
which are formed rebounding as they fall upon a cold sur-
face : this behaviour serves as a distinguishing test be-
tween amber and copal. When crushed amber is heated
in a retort it fuses at about 280° C. (536° F.), gives off
water, succinic acid, marsh gas, a mixture of liquid hydro-
carbons (known as oil of amber), and, finally, at a very
high temperature, a yellow substance having a 'wax-like
consistence. Sulphuretted hydrogen and other sulphur
compounds are also evolved in small quantity, for amber,
like several other fossil resins, contains a little sulphur
(sometimes J a part in 100) in organic combination.
Amber breaks with a conchoidal fracture. When frag-
ments of amber are being ground or powdered they emit
an aromatic odour. On being rubbed amber becomes
negatively electric in a high degree.
It is probable that true amber consists mainly of a
single resin (85 to 90 per cent, of the whole) represented
by the empirical formula wC^QH^gO. Small quantities of
two other resins which are soluble in alcohol and ether,
of a liquid hydrocarbon, and of succinic acid, are associ-
ated with the main constituent, which has received the
mineralogical name 'succinite.'
The classical names for amber were yjXeKTpov, lyncu-
rium, electnim, and sticcinimi. In early mediaeval times
amber was called vernix, a term which at first was applied
also to sandarac, and later in the fifteenth century to san-
darac only, when amber was designated as glas, or glassa.
In modern French amber is distinguished from ambre gvis
as amhre jaune, although it is also known as karabe and
72 COPAL
succin. It is the Bernstein of the Germans. The word
* amber ' is probably derived, through the Spanish, from
the Arabic anbar, a term applied to ambergris.
Copal is a name given to a number of hard resins which
vary not only in their degree of hardness, but also in their
degree of solubility : they are the produce of many dif-
ferent species, and even genera of trees, while the origin
of several of the kinds still remains unknown. One of the
hardest, palest, and best of all is known as Sierra Leone
copal, from the port of collection and shipment. It has
been identified as the resin produced by a tree, Copaifera
Gtiibouriiana, which belongs to the sub-order Caesalpineae
of the order Leguminosae. It is probable that the hard
West African pebble copal is the resin of the same tree,
but it occurs in rolled pebbles with an abraded surface,
and is at least semi-fossil : it is collected from the beds
of streams. Pebble copal has more colour than Sierra
Leone copal, but yields as strong a varnish. The latter
resin occurs in irregular rounded lumps or masses, gener-
ally varying in size from that of a hazel-nut to that of a
walnut. It is hard and elastic. It consists of at least
two resins, one of which, present to the extent of 33 per
cent., is soluble in absolute alcohol and in spirits of tur-
pentine. The other resin constitutes nearly the whole of
the remaining part of the copal, and becomes soluble in
most of the usual solvents, as well as in hot linseed oil,
when it has been previously heated to its melting-point
or to a temperature of 180° to 221° C. (360° to 430° F.).
Another process for rendering this and other kinds of
copal soluble is reduction to a fine powder in the presence
of water and the subsequent exposure of this powder to
the air for several months, or even a whole year. The
time requisite for this change may be shortened by keep-
COPAL 73
ing the powdered copal at a temperature higher than that
of the ordinary atmosphere. More will be said as to this
and other methods of increasing the solubility of copal in
the chapter on Varnishes.
Other species of the genus Copaifera yield similar but
inferior resins to that produced by C. Guibourtiana, but
C. Gorskiana is the source of Inhambane (near Mozam-
bique) copal ; Benguela copal, Angola copal, and Gaboon
copal are other sorts, varying in hue from straw-colour to
a dull reddish-orange, produced in all probabiHty by
different species of Copaifera. Much of the so-called
Manilla copal is the produce of Agathis loranthi folia.
Zanzibar copal is another hard and valuable resin of
African origin : it is often called anime. It is produced
by another leguminous tree, Trachylobium Hornemannia'
num, which belongs to the same sub-order, Caesalpineas, as
Copaifera. Most of this Zanzibar copal occurs in a fossil
or semi-fossil state in the earth near the roots of the trees,
or in places where the trees have formerly stood. This
fossilized resin is covered when dug up with a semi-
opaque, rough, and dull-brown crust ; when this powdery
coat is removed the remainder of the mass appears of a
transparent yellow colour, with a surface covered with
small rounded elevations like those on the rind of an
orange : this is spoken of as ' goose-skin.' Many of the
pieces are flat and tabular, with a thickness of a quarter
of an inch or more. The same resin, when occurring on
the bark of the living trees of the same species of Trachy-
lobimn, presents a smooth and glossy surface ; it is not so
hard as the fossil variety. Zanzibar copal melts at a
higher temperature than Sierra Leone copal, and is very
hard. In order to render it soluble it may be treated in
the same manner as the Sierra Leone copal. Its chemical
74 COPAL
nature requires further study. The varnish made with
Zanzibar copal, though darker in colour, must be re-
garded as at least equal in strength and durability to that
prepared with Sierra Leone copal.
A third resin, sometimes designated as copal, some-
times as anime, is produced by another leguminous tree,
Hymencsa cotirbaril, a native of Brazil and other countries
of South America. It is rather softer and more soluble
than Zanzibar copal. The copal of Madagascar comes
from another species of the same genus, H. verrucosa. A
Mexican copal is probably the resin of an allied species.
The resin from H. courharil is generally known as West
Indian copal ; fine specimens have been received from
Demerara.
The Bungo tree of Sierra Leone, Daniellia thurifera,
affords a resin of inferior quality. It is probable that the
same leguminous tree is the source of some of the Niger
and Sudan copals.
A rather hard resin of comparatively recent introduction
is Kauri or Cowdi copal, produced by the Cowdi pine of
New Zealand, Dammara australis. This is a coniferous
tree belonging to the tribe Araucariae. The largest masses,
some of them occasionally over lOo pounds in weight, are
found in the earth in many places far from those in which
the trees now grow. Kauri resin usually becomes more
transparent and yellower by keeping. It is generally
somewhat whitish, or streaked with opaque bands, when
first found. It is cleaned and scraped and then sorted
into several qualities. Great quantities were imported
into England for some years, and for a time it was largely
employed as the basis of most of the so-called copal
varnishes, on account of its abundance, its low price, and
its easy manipulation. But the varnish which it yields,
DAMMAR 75
or of which it constitutes the chief resinous component,
is inferior in hardness, toughness, and durabihty to that
made from Sierra Leone copal or Zanzibar copal.
Kauri resin is sometimes spoken of as dammar, but this
name properly belongs to the resins produced by other
trees, not by Damniava austvalis. White or Singapore
dammar is the resin of Damniava ovientalis. It is soft, and
may be scratched even by mica. ' Sal dammar ' is produced
by Shorea robustay the sal tree, widely distributed in India.
This resin, though soft, yields a good flexible paper
varnish. The tree belongs to the Dipterocarpeae. Vateria
indicay another Dipterocarp, yields piney resin or white
dammar : a similar resin is produced by another species,
V. acuminata^ a Ceylon tree. Several kinds of Hopea {H.
micrantha, H. odorata, etc.), which belong to the same
natural order, yield pale, transparent resins which are a
trifle harder than that of the sal tree. Black dammar or
Tinnevelly resin is produced by Canariiim stricUim ; it is of
very inferior quality. This tree belongs to the Burseraceae :
several kinds of elemi resin are also furnished by plants
belonging to the same natural order. These elemis are
soft, sticky resins, occasionally employed in varnishes to
prevent them from becoming brittle and cracking. They
contain essential oils and other aromatic bodies, and vary
very much in composition and properties, although they re-
semble one another in their solubility in boiling alcohol and
in their easy alterability. They are unsatisfactory resins.
The resin first known as sandarac was probably juniper
resin, although the name was also applied to amber. It is
spoken of by the older authorities on painting as having
a red colour. Its hue is a dull reddish orange, and it yields
a dark-brown varnish when dissolved by the aid of heat in
a drying oil. The effect of this varnish in imparting an
76 MASTIC
agreeable warm tone to pictures painted in tempera is very
evident, when the cold aspect of an old Italian unvarnished
tempera picture is compared with the glowing colour of a
painting which still retains its original sandarac varnish.
The resin now called sandarac is produced by another
coniferous plant [Callitris quadrivalvis), a native of Algiers.
It is a pale yellow resin, when fresh resembling mastic in
colour, but becoming yellower with age. It is brittle and
melts easily. When finely powdered and sifted it forms
one of the kinds of pounce used in preparing the surface
of parchment and vellum for writing and illuminating.
It dissolves in alcohol and in acetone, incompletely in
petroleum spirit and benzol.
There is one more resin which requires mention. This
is mastic. The best and most important sort of mastic is
produced by a small tree {Pistacia Lentiscus), belonging to
the cashew-nut order or Anacardiacese. This tree occurs
in Scio and other islands of the Greek Archipelago. Mastic
exudes in the form of tears from incisions made in the
bark. It occurs in small pea-like masses, and presents
when fresh a very pale straw-colour. It is so fragile that
it may be crushed to powder between the fingers. It has
an aromatic odour, and dissolves completely in boiling
alcohol and in spirits of turpentine. Its melting-point is
low, on an average about iio° C. It contains, besides its
resinous constituents, a small quantity of a volatile essen-
tial oil (a terpene) and of moisture. It yields a tender
but glossy varnish, largely employed for the final protec-
tion of pictures in oil. This varnish yellows with age,
and becomes fragile and fissured.
Resins, sometimes called mastics, are produced by other
trees of the same genus. These resins, which are of no
value for artistic purposes, are :
COPALS 77
Indian mastic from Pistacia cahdica.
Bombay mastic from P. Khinjak.
Pistachio mastic from P. Terebinthus.
In the following table, compiled from the results of
Bottler, are comprised some particulars concerning twelve
of the resins commonly called ' copals.' In the first
column the names are given, in the second the melting-
points, in the third the relative hardness (12 being the
hardest), and in the fourth and last column the degree of
solubility (xii being the least soluble). The specific
gravity of these copals depends so much upon the relative
freedom of the several kinds from cavities and bubbles
that the figures for this character are not included in the
table. It may, however, be mentioned that all these
resins are rather heavier than water, their specific gravi-
ties ranging from i '035 to i -07.
Name of Copal
Melting-point
Hardness
Solubility
Zanzibar -
- 275° C.
12
xii
Red Angola
- 315 -
II
ix
Pebble Copal -
- 230 -
10
xi
Sierra Leone -
- 195 ■
9 -
V
Yellow Benguela
- 180 -
8 -
vii
White Benguela
- 185 -
7 -
X
Congo
- 190 -
6 -
iv
Manila
- 145 -
- 5 -
ii
White Angola -
- 245 -
4 -
i
Kauri
- 150 -
3 -
iii
Demerara
- 90 -
2 -
viii
Brazilian -
- 95 -
I
vi
All these numbers are approximative only ; and the
valuation of these resins for varnish-making ought to
take into account other properties not here recognised,
such as toughness after as well as before heating and
solution, liability to darkening in colour by lapse of time,
and durability.
78 VENICE AND STRASBURG TURPENTINE
Turpentines, Oleo-Resins, and Balsams. — There is a group
of substances, many of them derived from coniferous
plants, which are, or have been, included under the term
balsam. Strictly speaking, this designation should be
limited to those resinoid exudates which contain benzoic
or cinnamic acid, while the term oleo-resin, or, better still,
the term turpentine, should be given to those soft and semi-
liquid natural exudates which consist of terpenes asso-
ciated with bodies of resinous character. The word
turpentine is, however, so generally connected with the
volatile hydrocarbons (terpenes) distilled from these
exudates, that it should be clearly understood that the
three vegetable products hereunder described are of
natural, not artificial, origin. It is in consequence
of the preservative influence upon certain pigments
which has been assigned, not without experimental
confirmation, to these bodies that they are noticed
here.
Venice Turpentine. — Under this name the resin of the
common larch is now known. It comes chiefly from Tirol.
Recent examination has shown it to consist mainly of three
groups of compounds, namely, about 63 per cent, of
resinous acids, 20 per cent, of terpenes, and 14 per cent,
of resins. The best specimens are never quite so clear
and free from colour as those of the next product to be
described.
Strasburg Turpentine is derived from Abies pectinata, the
silver-fir, the best quality coming from the Italian side
of the Tirolese Alps. It contains about 57 per cent, of
resinous acids (not identical with those in larch-turpentine),
28 per cent, of terpenes, and 13 per cent, of resins. This
turpentine is the true Olio d'Abezzo of Italian writers, and
when dissolved in a terpene was used as a varnish for
WAX 79
pictures in tempera and oil, and for the special protection
of verdigris and of some other dangerous pigments. Some
specimens of this turpentine are beautifully clear and
colourless. It is decidedly superior to larch turpentine,
with which it has no doubt been often confused, and to
Bordeaux turpentine, obtained from Pinus Pinaster. The
chemical study of the olio d'abezzo has at present thrown
no light upon the specially protective or locking-up
qualities which are claimed for this turpentine.
Canada Balsam^ from Ahies halsamea, much resembles
Strasburg turpentine. The resinous acids which consti-
tute three-fifths of its weight are said to be different from
those already mentioned as existing in the turpentines
from the larch and silver-fir.
Wax. — The true waxes, unlike the oils described in
Chapter V., are not glycerides, and do not therefore yield
glycerin when they are saponified — that is, turned into
soaps by the action of alkalies. Ordinary beeswax is the
best known, and probably the most important of all the
different kinds ; but very few experiments have been made
as to the utiHzation of exotic and vegetable waxes in the
processes of painting. Crude beeswax requires purifica-
tion and bleaching in order to fit it for artistic use. The
first operation consists in melting the wax at nearly the
lowest temperature possible, and then pouring it in a
slender stream into a cold saturated solution of alum,
agitating the latter all the time. The granulated wax thus
prepared may be bleached by exposure for several days on
linen cloths to the action of the sunlight and dew; or it may
be treated with dilute chromic acid solution, or with hydro-
gen peroxide. All these processes succeed better when
the wax is in the form of thin sheets or ribbons. The
bleached wax, after thorough washing and drying, is to
8o WAX
be re-melted. Its hardness is increased and its melting-
point raised by the above treatment.
Bleached beeswax melts at 62° or 64° C. (144° or
147° F.). It consists of four distinct substances, not
present in all samples in the same proportions. By boiling
wax with strong alcohol a substance called myricin
(myricyl palmitate) is left undissolved. The dissolved
portion is the larger ; the bulk of it, which crystallizes
out as the alcohol cools, was formerly called cerin. It is
a mixture of two fatty acids. The cold alcohol still
retains a small quantity of a fourth substance.
Beeswax, by long-continued exposure to atmospheric
influences, disintegrates and partially perishes by oxida-
tion. It is a constituent of Gambier-Parry's spirit-fresco
medium, into which it is introduced in order to impart a
matt appearance to the painting. Excellent examples of
the use of melted wax as a binding material for pigments
may be seen in the National Gallery and the Victoria
and Albert Museum. They are encaustic portraits, exe-
cuted probably in the second and third centuries of our
era, and were discovered by Professor W. M. Flinders
Petrie, in the Hawara Cemetery, Fayum, Egypt. The
pigments were mixed with wax and laid on in the melted
state. The wax having become disintegrated in the
course of centuries has been re-melted, some fresh wax
having been added in several instances.
Wax is abundantly distributed in the vegetable world ;
its production is, in many cases, stimulated by the
attacks of insects. Thus, Chinese wax is produced by
the puncture of Coccus Pela, living on Ligustrum lucidum
and Fraximis chinensis. Chinese wax, which melts at
82° C. (180^ F.), consists almost entirely of cerotyl
cerotate. Brazilian or Carnaiiba wax occurs naturally
PARAFFIN WAX 8i
in thin films on the leaves of a palm {Copernicia cerifeva) ;
it melts at 84° C. (183° F.). Japanese or Ibota wax is
probably produced by the attacks of a coccus on Ligustvum
Ibota; it melts at 42° C. (108° F.).
Paraffin wax, hard paraffin, solid paraffin, and ceresin,
are names given to certain mixtures of hydrocarbons
occurring in native petroleum and in the * mineral wax '
called ozokerite, and also in the tars produced by the
destructive distillation of wood, peat, lignite, bituminous
shales, and coals. The liquid hydrocarbons which
accompany the paraffin wax are described so far as
necessary in Chapter XI. under the head of Solvents.
Paraffin wax, so far as its main or fundamental con-
stituents are concerned, contains no oxygen, and is a
mixture of several of the least alterable of all organic
compounds ; very few chemical reagents have any action
at all upon it. On this account it presents for artistic
purposes a marked superiority over beeswax or any
vegetable wax. Of the hydrocarbons occurring in large
quantity in paraffin wax the best known are those to
which the chemical formulae CggH^g, C24H50, CggHgg,
C27H56, CggHgg, and C^qHqq belong. The melting-point
of paraffin wax oscillates within wide limits, say, from
30° to 80° C. The higher the melting-point the harder,
the heavier, and the less crystalline is the material. For
artistic purposes, hardness and the absence of a tendency
to separate from solution in the form of large crystals
are desirable properties. Unfortunately the hardest
paraffin waxes of high melting-point are much less
soluble in oils, terpenes, and varnishes than the softer
varieties, and thus their usefulness is limited ; they are
also somewhat yellowish in hue. I have, however,
found that a pure paraffin wax from the Bathgate shale,
6
82 PARAFFIN WAX
having the melting-point of 65-5° C. (150° F.), answers
every purpose. It is sufficiently hard and but indistinctly
crystalline, and yet may be dissolved in fair abundance
by the usual solvents. It is convenient to preserve it
for use in the form of small flattened globules, which
are easily prepared by melting the substance and pouring
it drop by drop on to the surface of a large sheet of glass
previously moistened by breathing upon it. When these
drops are shaken in a bottle they rattle like small pebbles,
and do not mark the glass ; when the softer soHd paraffins
are thus treated, they fall with a thud, and leave streaks
and spots upon the interior surface of the vessel. This
difference of deportment affords a ready means of dis-
tinguishing between a paraffin wax suitable for artistic
uses and one which had better be rejected.
The manufacture or isolation of hard paraffin and its
purification are not described here. The processes em-
ployed — distillation, treatment with oil of vitriol, frac-
tional crystallization from solvents, etc. — involve the use
of complex apparatus. It may, however, be here stated
that commercial hard paraffins vary somewhat in purity.
Those obtained from mineral wax or ozokerite are nearly
free from oxygen compounds ; while those derived from
the products of the destructive distillation of shales, coal,
etc., sometimes contain as much as 3 per cent, of oxygen,
indicating the presence of other bodies besides hydrocar-
bons. Some of these bodies are of an acid nature ; these
may be separated by repeatedly boiling the commercial
paraffins in question with a 5 per cent, solution of caustic
potash. The following table shows the relations subsist-
ing between the melting-point and the specific gravity
(at 20° C.) of six different samples of hard paraffin,
generally known as ceresin, from ozokerite :
PARAFFIN WAX
83
No. oj
Melting-
Specific
No. of
Melting-
Specific
Sample
point
Gravity
Sample
point
Gravity
I -
- 56° c. -
- 0-912
4 - •
72° C. -
- 0-935
2 -
- 61° - -
- 0-922
5 -
76° - -
- 0-939
3 -
- 67° - -
- 0-927
6 - -
82° -
- 0-943
A sample of ceresin made from ozokerite was furnished
to meat my request by the late J. Calderwood, of Price's
Patent Candle Company. It has a setting-point of about
156° F. (69° C), and is almost non-crystalline in appear-
ance. It possesses, however, a somewhat greasy feel and
a slight yellowish hue. I find by experiment that this
ceresin, with a small admixture of a refined paraffin
(from the same manufacturers) having a melting-point
of 147° F., forms an excellent substitute for the hard
paraffin wax (melting-point 150° F.), from Bathgate shale,
described on a preceding page, and unfortunately no
longer to be met with in commerce.
Hard paraffin wax may be used in the preparation of
painting mediums as a substitute for beeswax ; for pre-
venting the separation of heavy pigments, such as ver-
miUon, from the oil in which they are ground ; and for
the preparation of certain painting-grounds.
U In practice it has been found that far smaller quan-
tities of beeswax than of paraffin wax are required to
prevent the subsidence of heavy pigments from the oil
in which they have been ground. Moreover, the working
of paints containing a small quantity of beeswax is more
agreeable than is the case with those into which paraffin
wax has been introduced.
CHAPTER VII
YOLK AND WHITE OF EGG ; SIZE ; GLUE
The materials described in the present chapter owe their
peculiar properties — at least, in great measure — to the
presence of chemical compounds which contain the ele-
ment nitrogen. Now, this element is not a constituent of
any of the artists' materials already described, nor, indeed,
of any others, except a few pigments, such as aureolin,
Prussian blue, and indigo. The presence of nitrogen in an
organic compound is very often accompanied by a measure
of instability, or proneness to change; the nitrogenous con-
stituents of eggs, and of size, afford illustrative examples.
Another source of weakness in the composition of the
nitrogenous constituents, both of the white and of the yolk
of eggs, lies in the presence of another element — namely,
sulphur. Part of this sulphur readily leaves the original
substance, yielding simpler compounds, such as sulphur-
etted hydrogen, and ammonium sulphide, which possess
the objectionable property of discolouring many of the
metallic pigments used by artists. On the other hand, all
these nitrogenous bodies are susceptible of coagulation,
whereby they become insoluble, and very much less prone
to change. Indeed, the majority of them may be turned
into a substance which is virtually leather, a material
which resists decay in the most marked manner. This
84
WHITE OF EGG 8$
tanning operation may be readily effected by treating the
substances in question with a solution containing tannin,
the active ingredient of oak-bark, sumach, nut-galls, etc.
We will first consider the composition of the yolk and
white of ordinary hen's eggs. The percentage proportions
are, on the average
v->
Yolk
White
Water -
-
- 51-5 -
-
- 84-8
Albumen, Vitellin,
etc.
- 15-0 -
-
- I2'0
Fat or Oil -
-
- 22 -o -
-
0-2
Lecithin, etc.
-
- 90 -
-
- trace
Mineral Matter -
-
i-o -
-
- 07
Other Substances
-
- 1-5 -
-
- 2-3
The white, it will be seen, is characterized by the
presence of 12 parts per hundred of albumen, which is in
solution in the ropy Hquid. When this solution is heated
to a temperature considerably below that of boiling water,
the albumen becomes insoluble, and is said to be coagu-
lated ; it is not capable of being again dissolved in its
original menstruum. Solutions of tannin, corrosive subli-
mate, and many other compounds, inorganic and organic,
produce a similar effect. But egg-white is not a pure
solution of albumen. For all practical purposes in the
arts, it may be sufficiently freed from extraneous matters
in the following manner : The necessary number of
* whites ' are mixed in a wide-mouth stoppered bottle, with
twice their bulk of water, and shaken up thoroughly ;
then a slip of yellow turmeric-paper is dropped into the
mixture. Drop by drop weak acetic acid is poured in,
until the reddened turmeric-paper ha.sj7Lst, or nearly, re-
gained its original yellow hue. In this way the alkaline
reaction of the liquid is almost neutralized, and it becomes
thinner. After further agitation, the mixture is poured
upon a piece of well- washed muslin in a funnel. The
86 COAGULATION OF ALBUMEN
clear liquid which drops through has been freed from
membranes, etc., and contains nearly 4 per cent, of albu-
men. It may be concentrated by cautious evaporation
at a temperature not exceeding 50° C. The albumen
which it contains is a very complex substance, containing,
besides carbon, hydrogen, nitrogen, and oxygen, about
1*6 per cent, of sulphur. A solution of albumen spread
upon glass, and allowed to dry slowly at the ordinary
temperature, leaves a residue of albumen in the form of a
nearly-transparent film. This, when quite dry, is brittle,
and easily cracks. If, before it be quite dry, it be heated
to 70° or 75*^ C, it cannot be again dissolved by water,
having been converted into the insoluble form. In this
condition it is much less prone to change. It will now
be seen how powdered pigments, if ground up with albu-
men solution and then used in painting, may be made to
cohere, and also to adhere to the painting-ground of cloth,
paper, or plaster, on which they have been spread. And
afterwards, by simply heating the work sufficiently, the
whole coloured layer may be rendered insoluble and irre-
movable by water. Advantage may also be taken of the
action of tannin on albumen to secure the same result
— the coagulation of the albumen. We may coat a piece
of fine linen cloth with albumen-solution, and before it is
quite dry we may paint upon it with pigments which have
been previously ground up with a weak solution of tannin.
If the work be carefully done, the colours will, when dry,
be found to have been fixed by the reaction between the
tannin and the albumen. If, however, the pigments be
laid on somewhat quickly, it may be found necessary to
give the whole surface a final coat of albumen-solution.
We have dwelt at some length upon this employment of
tannin, or of heat, to secure the coagulation of albumen,
YOLK OF EGG 87
because it serves to illustrate the way in which paintings,
executed with egg-yolk, or size, as a medium, may be
fixed. For, as we shall now proceed to show, egg-yolk
and size possess many characters in common with
albumen-solution.
But the yolk of an egg contains other substances besides
albumen. First of all, the albumen present is accompanied
by another similar compound called vitellin, which re-
sembles it in composition and properties, and which, for
our present purpose, we need not further describe, except
so far as to state that, unlike albumen, it is not soluble in
water. Of albumen and vitellin, taken together, egg-yolk
contains, as we have seen, not less than 14 or 15 per cent.
But egg-yolk is something more than a solution of these
two similar bodies. It is, in fact, an oily emulsion, in which
innumerable minute globules of a thick, fatty oil are sus-
pended in an albuminous solution. And, moreover, the
amount of this oil is large ; there is about 22 per cent of
it, and associated with this oil there is no less than 9 per
cent, of a curious compound called lecithm, which has
many of the physical properties of a fat. It seems to be
a triglyceride, including two fatty-acid radicles and one
phosphoric acid radicle. Associated with lecithin there is
a nitrogenous basic compound. Although lecithin re-
sembles oils and fats in its behaviour to most solvents, it
yet differs from them in this one particular, that it is very
hygroscopic and swells up in water, forming a kind of
emulsion. Now, 9 parts of lecithin with 22 parts of oil
make up nearly one-third of egg-yolk, or 31 parts of oily
or fatty matter per 100, as against 15 parts of albuminoid
matter, or vitellin and albumen taken together. Hence
it happens that egg-yolk, the usual vehicle for pigments in
the best kind of tempera-painting, must be regarded as
88 SIZE AND GLUE
essentially an oil-medium. As it dries, the oil hardens,
and remains intimately commingled with the albuminous
substances left behind on the evaporation of the water
present. These albuminous substances coagulate and
become insoluble in the lapse of time — a change greatly
accelerated by the old practice of exposing the finished
tempera picture to sunshine previous to varnishing it.
Size and glue may be considered together. They consist
of two distinct yet similar compounds, known respectively
as gelatin and chondvin. These bodies consist of carbon,
hydrogen, nitrogen, and oxygen ; and, when pure, they
contain no sulphur. They are soluble in hot water, yet
are coagulable by tannin and by some other compounds,
organic and inorganic. Chondrin is thrown down from its
solution by alum, and, indeed, by several compounds which
do not precipitate gelatin. The latter body is obtained
from skin, tendons, and bones. These organized structures
contain a substance called ossein, or collagen, which, under
the influence of boiling water, dissolves, becoming changed
into gelatin. This conversion occurs more quickly when
the process is performed under a pressure somewhat
greater than that of the atmosphere, and, therefore, at a
temperature rather higher than ioo° C. In this way the
transformation of the organic tissue of ivory, bone, vellum,
parchment, fish-bladder, etc., into gelatin may be readily
effected. The purity of the product depends, in part, upon
the care with which the raw materials have been selected
and cleansed, in part upon the temperature and the dura-
tion of the extraction. If the temperature be too high, or
the boiling be much prolonged, the gelatin produced is
transformed partially into a substance which does not
gelatinize when its aqueous solution is cooled, Chondrin
is obtained from cartilage, which consists mainly of carti-
SIZE AND GLUE 89
lagin, or chondrigen, by the same process which changes
collagen into gelatin. A hot solution of chondrin gelatin-
izes on cooling just like one of gelatin ; but it does not
yield, with the same amount of substance, so firm a jelly.
Size, glue, and commercial gelatin, consist of mixtures of
gelatin, chondrin, and the non-gelatinizing substances
produced by the long-boiling or the over-heating of their
solutions. Isinglass, vellum, and ivory-dust yield a size
which contains nothing but gelatin and a little mineral
matter ; the darker and stickier kinds of glue contain
many impurities, having been made from very varied
materials, such as ox-hoofs, horseflesh, old leather, etc. ;
they often contain sulphuric acid.
In selecting a size for artistic use, the special purpose
in view will indicate whether an insoluble (in cold water)
and strongly-gelatinizing, or a partially soluble and very
adhesive one should be selected. The former is less liable
to crack when dry than the latter. The very fine gelatins
used in photography will often be found suitable. A few
experiments, with cold water and then with hot, will soon
reveal the peculiarities of the samples submitted to ex-
amination. As caustic lime, caustic soda, chloride of lime,
sulphurous acid, and certain mineral acids, are frequently
employed in the manufacture of size, glue, and gelatin, it
is absolutely necessary to ascertain, before using these
materials in any process of painting, their freedom from
free acids, free alkalies, or bleaching agents. A hot-water
solution of the material must not redden blue litmus-paper,
nor bleach dahlia-paper, nor embrown tumeric-paper.
Glue and size may sometimes be purified and improved
by cutting up the solid or gelatinous mass into small
pieces, soaking them in distilled water for a few hours,
and then pouring off the liquid before dissolving them .
90 PRESERVATION OF EGG-YOLK
The temporary preservation from putrefaction of the
solutions of the substances described in the present
chapter, may be effected in several ways. A lump of
camphor, or a few drops of eugenol (from oil of cloves),
is generally sufficient. I have preserved the egg-yolk
medium for tempera-work for many days in an agreeable
condition for use by the following plan : A saturated
solution of eugenol in 5 per cent, acetic acid is first made,
then this is added, drop by drop, with constant agitation,
to the required number of yolks in a wide-mouth bottle,
the point at which to stop further addition being learnt by
the change of colour of a slip of turmeric-paper. When
this paper just regains its original yellow colour, which
was turned brownish-red by the yolks, no more acetic
acid is wanted. Any water needed for thinning the
medium may now be added, together with a lump of
camphor, which will remain floating on the surface.
CHAPTER VIII
GUM, STARCH, DEXTRIN, HONEY, AND GLYCERIN
The term gum is properly applied to a number of non-
crystalline, structureless substances, of vegetable origin.
They consist essentially of so-called hydrates of carbon,
and are either soluble in cold water, or swell up when left
therein for some time. The only gum of any importance
in painting is gum-arabic. This name is not, however,
exclusively applied to one variety only ; it is given to the
gums which exude from several species of Acacia. For
instance, Acacia arabica furnishes the Morocco, Mogador,
Brown Barbary, and East Indian gums of commerce.
But it should be noted that, although A . arabica is a native
of India, and is grown to some extent in many parts
of that empire, the gum it yields is rarely, if ever, exported
thence, the so-called East Indian gum-arabic being really
taken from Red Sea ports to Bombay, and thence re-
shipped to Europe. Acacia arabica, however, does not
furnish a strong and durable gum, and it is from another
species, A. Senegal, that we obtain the gum employed as a
binding material for water-colours. This gum is known
commercially as Kordofan, picked Turkey, white Sennaar?
and Senegal gum. The tree which yields it is a native of
Senegal and the Sudan ; it grows to a height of twenty
feet. The supplies which come from Kordofan are of the
91
92 GUM SENEGAL
finest quality, but all the grades of gum from A . Senegal
are superior to the produce of A . arabica in their greater
dryness, density, and adhesiveness, as well as in the
smaller amount of mineral matter which they contain. It
may be added in this place that, according to some authori-
ties, a part of the gum Senegal of commerce is produced
by other species of Acacia besides A. Senegal, such as
A . Adansonii, A . albida, A. dealbata, A . nilotica, A. Vevek, etc.,
and even from species of Kaya, Spondias, and Stevcidia.
Suakim gum, the produce oi Acacia stenocavpa and A. Seyal,
varies greatly in quahty, but is largely imported into
England, and much used, though not generally available
for fine work.
Gum consists mainly of arabin, a mixture of the salts
of an acid called arable acid, and of the free acid itself.
The salts are those of the three bases — potash, lime, and
magnesia ; water is also present. It is probable that, in all
varieties, even of the finest gum Senegal, other organic
acids, besides arable acid, are present. An analysis of a
fine specimen of picked ' Turkey gum ' gave 15 per cent,
of water, and 2*8 per cent, of ash, leaving 82-2 per cent,
for the arable and other allied acids and organic matters.
The arable acid was formerly expressed by the formula
^i2^22^iij ^^^ ^^^ experiments of O'Sullivan indicate a
much more complex composition (CggH^^gOyJ.
Gum from Acacia Senegal, the only sort which ought to
be employed in painting, should be nearly free from colour,
and should dissolve in cold water without leaving an appre-
ciable residue. Its watery solution should be clear, and
should give no colour with tincture of iodine, but an abun-
dant precipitate with ammonium oxalate solution. If
iodine produce a purplish colour, adulteration with dextrin
is indicated ; the white precipitate thrown down by the
GUM TRAGACANTH 93
oxalate shows the presence of calcium, a constant con-
stituent of the genuine gum. I have found that the
samples of gum sold to me as gum Senegal were of a more
pronounced yellowish colour than those bought as gum-
arabic and best Turkey : the lumps varied more in size,
often contained air-bubbles, and were less fissured. The
adhesiveness and toughness of these samples, moreover,
compared favourably with these properties as exhibited
by the finest and whitest ' Turkey gum ' obtainable.
For the preparation of water-colours, and for occasional
use in the operations of painting, it is convenient to have
at hand a standard solution of gum. This may be pre-
pared by dissolving i ounce of the selected gum reduced
to fine powder in 2 measured ounces of boiling distilled
water. The powdered gum should be very slowly added,
with constant stirring, to the boiling water. When the
whole is dissolved, the liquid is allowed to stand for at
least a day ; then it is decanted from any sediment that
may have been deposited into a wide-mouth bottle with-
out cork or stopper, but covered with a glass cap. It is
well to allow a lump of camphor to float in it, or to add
to it a couple of drops of eugenol, the active antiseptic
constituent of oil of cloves : a still more effective pre-
servative is /5-naphthol.
Gum tragacanth is produced by certain leguminous
shrubs belonging to the genus Astragalus. Amongst these
may be named: A. giimmifer, A. eviostylm^ A. hvachy calyx,
and A. adscendens. Its constituents include a small quan-
tity of a gum soluble in cold water, a little starch and
cellulose, and a large proportion of a mucilaginous body
which swells up in cold water, but does not dissolve.
The substance having these properties is a compound of
carbon, hydrogen, and oxygen of very complex consti-
94
STARCH
tution, which has been called hassorin. Gum tragacanth
contains from 12 to 15 per cent, of water, and leaves 2 to
3 per cent, of ash when burnt. A mucilaginous medium
made with gum tragacanth may be used for painting on
linen : it is not very easy to prepare so as to be of uniform
consistency. A fairly good plan is to place the finely-
powdered tragacanth in a bottle, and to add enough spirit
of wine to moisten it : then add the required amount of
water, and shake the mixture gently at intervals. Water
containing no more than 3 or 4 per cent, of the gum con-
stitutes a moderately thick mucilage.
^ Tragacanth mucilage containing from |^ to 2 per cent.
of this gum serves as a binding medium in the making of
crayons for pastel work.
Other gums are of small importance. They commonly
contain much bassorin and but little arabin. The Aus-
tralian wattle gums from several species of Acacia are
perhaps thus constituted ; but if this be the case, the
bassorin present in them seems to present some points of
difference from the bassorin of tragacanth. Cape gum is
produced by Acacia horvida : it is inferior to gum-arabic,
as a substitute for which it is used in Cape Colony.
Starch comes next in our list. This important food-
substance occurs in commerce in a condition so nearly
pure that there is no need to describe its character. For
the limited uses to which it is put in artistic practice the
uncoloured or white starch should be selected. The
starch from rice, wheat, maize, or potatoes may be em-
ployed indifferently. Arrowroot may also be used. The
preparation of starch-paste does, however, require some
care. The best plan is to thoroughly agitate 50 grams of
the dry powdered starch with enough cold water to produce
a liquid of creamy consistence, and then to pour this
STARCH 95
mixture slowly into a vessel in which about 300 cubic
centimetres of distilled water is kept in steady ebullition.
All but 2 per cent, of the starch will dissolve into a nearly
transparent homogeneous paste : the quantity of starch
must be reduced if a thin liquid be required.
^ Although starch has not hitherto been much employed
in painting, its merits are such as deserve a more extended
use. As its constituent elements include neither nitrogen
nor sulphur, it is, on the one hand, more stable and less
liable to the attacks of micro-organisms than size, white
of egg^ or casein ; while, on the other hand, its chemical
inertness is such that there is no fear of its exercising
any injurious effect on colouring matters. But ordinary
starch paste, owing to its viscous character, is not very
suitable as a binding material for pigments. However,
by means of certain treatments, as with ozone, glycerin,
or volatile acids, starch can be brought into a more soluble
and Uquefiable form of great adhesiveness, and admirably
fitted as a binding material in water-colour painting.
Moreover, the various preparations of soluble starch
possess in a high degree the property of becoming in-
soluble in cold water after they have once become dry.
In consequence, a pigment laid on in admixture with a
soluble starch vehicle becomes, after it is dry, irremovable
by water, so that further washes of colour may be added
without disturbing the previous layers.
U Soluble starch may be obtained by dissolving 10
grams of caustic alkali in 400 cubic centimetres of water,
and then stirring in 100 grams of starch previously ground
into a paste with a little water. The mixture should then
be carefully and uniformly warmed until it has become
transparent. After heating for about fifteen minutes,
hydrochloric acid is added to the paste until it no longer
96 STARCH
shows an acid reaction to litmus paper : the addition of
a little ^-naphthol will protect the product from mould.
Similar preparations of starch can be bought under
various fancy names (such as vegetable glue). They are
produced in the way just described, and are used in the
preliminary priming of canvas instead of ordinary animal
size ; but before the artist employs any of them, the sample
should be tested with reddened litmus paper to see that
it has no alkaline reaction, and with blue litmus to learn
if an excess of acid be not present. For the careful and
necessary neutralization of the product is not unfrequently
omitted.
The drawback to the preparation of soluble starch by
treatment with caustic alkali lies in the presence of much
alkaline chloride in the product ; it is not desirable to in-
troduce sodium chloride, and still less potassium chloride,
into a coloured drawing. An entirely satisfactory variety
of soluble starch is obtained by the limited action of fresh
malt-extract, in very small quantity, upon starch-paste
at 75°. Or dilute sulphuric acid may be used, in the
same way, to produce the desired transformation, the
action being stopped directly the liquid becomes clear,
by stirring in an excess of precipitated barium carbonate,
which is subsequently removed by filtration. Still another
method of preparing soluble starch is by heating it with
glycerin. It is recommended to employ 6 grams of dry
potato starch and loo grams of glycerin, heated together
for about half an hour to 190° C, and then cooled down
to 120** C. The soluble starch may be thrown down
from this liquid by adding to it three times its bulk of
strong alcohol. It must be remarked that the so-called
* soluble starch ' prepared by the several methods just
described is not precisely an identical product. In its
HONEY 97
most characteristic form it dissolves freely in hot water,
but is deposited as a white powder during the cooling of
the solution ; cold water holds about 3 per cent, in solu-
tion. It is stated that the variety prepared by means of
sodium peroxide is much more soluble in cold water.
Starch contains carbon, hydrogen, and oxygen only,
and is a carbohydrate having the empirical formula
wCgHj^Og. It is a stable compound. Commercial starch
always contains some water, generally from 12 to 18 per
cent.
Dextrin, or British gum, as met with in commerce, is
prepared from starch in one or other of several different
ways, and is a variable mixture of at least three varieties
of true dextrin, soluble or modified starch, starch, a sugar
called maltose, and certain minor ingredients and im-
purities. It will suffice for the purpose now in view, if
we select a commercial dextrin, free from acidity, dis-
solving nearly completely in cold water, and then yielding
a solution which, even when strong, has only a light
yellowish or brownish colour. When a filtered cold-water
solution of commercial dextrin is allowed to evaporate on
a glass plate, and the residue becomes air-dry, the film
of dextrin left differs from one of true gum by being less
friable. A solution of dextrin is, however, far less ad
hesive than one of true gum of the same strength.
Honey now claims our attention. It is a common in-
gredient in moist water-colours, and was often employed
in size-painting. It is used to counteract the brittleness
of gum or of size when dry, or, by its absorption and
retention of water, to keep a paint moist. Honey con-
sists of nearly equal quantities of two sugars known as
dextrose and laevulose, a Httle sucrose or common sugar,
small quantities of non-saccharine compounds, and about
7
98 GLYCERIN
20 per cent, of water. As the useful properties of honey
depend entirely upon its laevulose, a solution of this sugar
should be employed instead of the raw honey : this may
be easily prepared in the following way : Pure pale honey,
kept until it has become crystalline and semi-solid from
the separation of dextrose, is mixed gradually with four
times its bulk of proof spirit, and thoroughly shaken at
intervals for a few hours. The pale yellow alcoholic solu-
tion is then filtered : the filtrate is a solution of laevulose,
accompanied by small quantities of the other sugars of
honey and of harmless impurities, and for some artistic
urposes is at once available. Should it be desired to
obtain a more concentrated solution of this substance, the
liquid may be evaporated to the desired consistency in
a porcelain basin, or it may be submitted to distillation
in a retort. The aqueous solution of laevulose may be
decolourized by filtration through warm animal charcoal.
Laevulose, when free from water, forms a glassy solid ;
but it is usually obtained as a thick syrup. Although
this sugar is capable of assuming the crystalline form, it
never does so under ordinary conditions. It has a strong
attraction for moisture ; on this property its usefulness
as a constituent of certain paints depends.
Glycerin was discovered in 1779 by Scheele as a by-
product in the preparation of lead-plaster ; for a long time
the comparatively small quantity of glycerin met with in
commerce was obtained in this way. It is now prepared
from oils and fats by distilling them in a current of super-
heated steam, sometimes by first saponifying them with
alkalies, or decomposing them with sulphuric acid, and
then submitting them to this distillation treatment.
Glycerin generally occurs as a thick syrup with a sweet
taste : when pure, it may be obtained in deliquescent
GLYCERIN
99
crystals. Its empirical formula is CgHgOg. It is a
strongly hygroscopic or water-attracting substance, the
pure water-free glycerin being capable of absorbing more
than one-third its weight of water from the air. Com-
mercial glycerin ahvays contains water : the specific
gravity of the liquid affords a rough method of estimating
the amount. For pure glycerin at 15 '6° C. has the specific
gravity 1*265, while that which contains 20 per cent, of
water is reduced to i'2i3; with 30 per cent, it is i-i86,
and with 40 per cent. i'i57. The presence of sugar,
a not uncommon adulterant, may be recognised by the
turbidity caused by mixing the glycerin, after evaporation
to remove water, with chloroform. Glycerin containing
lead darkens when sulphuretted-hydrogen water is added
to it, while the presence of acids may be recognised by
blue litmus-paper, which is not reddened by pure glycerin.
The water-attracting property of glycerin induced me
to use it as a substitute for honey in preparing moist
water-colours so long ago as 1856, but I am given to
understand that it was employed in 1847 by Messrs.
Winsor and Newton. Even in cake-colours a trace of
glycerin may be introduced with advantage, as it renders
them less friable and more easily rubbed down with water.
It prevents size, glue, and white of egg from becoming
brittle on drying, and on this account may be used in the
preparation of linen, canvas, etc., as painting-grounds.
Care must, however, be taken in every case not to add
more glycerin than is necessary to effect the purpose in
view. It is a useful addition to gum-water, i dram to
each ounce of gum present being sufficient ; some copying-
inks contain it. Modelling clay may be kept moist by
means of glycerin.
CHAPTER IX
WATER-GLASS, LIME- AND BARYTA-WATER
The name water-glass appears to have been first applied
to those silicates of potash and of soda which are soluble
in water by Professor J. N. von Fuchs, in 1825; but
Glauber, so early as 1648, made a soluble potash silicate,
which he termed fluid silica. Van Helmont had prepared
a similar compound in 1640- The actual manufacture on
a commercial scale of these salts dates, however, from
1825 only, and the credit of originating their production
belongs to Von Fuchs. They differ from the compounds
constituting ordinary and insoluble glass by containing
no lime, baryta, alumina, or other earthy base. They
are made in several ways. The purest sand obtainable
is fused with carbonate of potash, or carbonate of soda,
or a mixture in the desired proportions of these two car-
bonates, in the presence of a little powdered charcoal.
The fused mass dissolves by long continued boiling in
water, and yields a heavy syrupy liquid of strongly alka-
line reaction. By evaporating this liquid to dryness, and
fusing the residue, the water-glass may be obtained in a
solid form, and then closely resembles ordinary glass in
appearance. Water-glass may also be made by heating
flints red-hot, quenching them in water, and then digest-
ing the powdered siHca thus obtained with soda-lye or
potash-lye under pressure.
100
WATER-GLASS loi
Three kinds of water-glass have been used in water-
glass painting or stereochromy. One of these is a potash
silicate, another is a soda silicate, the third is a mixture
of these two, or a potash-soda silicate, called double water-
glass. The solutions of the two former silicates as met
with in commerce vary a good deal in their relative pro-
portions of silica and alkali ; it is not desirable that they
should contain so much silica as was recommended in
the original papers of Von Fuchs, the inventor of stereo-
chromy, and of Kuhlmann, who subsequently modified
the process. Indeed, it has often been found useful to
add a little pure caustic potash or caustic soda-solution
or ammonia to the commercial solutions of water-glass
before diluting them with distilled water for use in this
process of painting.
A solution of water-glass, if allowed to dry upon a piece
of ordinary glass, leaves an opaque white irremovable
stain. Water-glass alters or destroys, in virtue of its
strong alkalinity, the great majority of organic pigments.
On the same account it cannot be used with flake-white,
aureolin, the chromates, vermilion, and several other
mineral pigments. It hardens zinc-white, some of the
ochres, earths, and terre verte, forming with them, or
with some of their constituents, double silicates, which
are quite insoluble in water. The fixative power of water-
glass in stereochromy depends indeed mainly upon actions
of this order which occur between it and ingredients of
the plaster or painting-ground, and of the pigments. It
was formerly supposed that when an alkaline silicate acted
upon carbonate of lime a double decomposition occurred,
of v/hich the only products were an alkaline carbonate,
and lime silicate. But subsequent investigation has
proved that the change in question is more complex,
I02 LIME-WATER
a considerable quantity of a double and insoluble silicate
of lime and alkali being produced. Similar double silicates
of potash or soda and zinc, of potash or soda and baryta,
and of potash or soda and alumina, have been proved to
exist in stereochromic work ; doubtless many others are
also present. They are not only insoluble in water, but
are harder than the materials out of which they have
been formed.
Commercial solutions of water-glass contain from 28 to
60 per cent, of the alkaline silicate or silicates. They
should be carefully preserved from access of air, the
carbonic acid of which produces much alkaline carbonate
(often separating in crystals in the case of soda), and finally
causes the separation of gelatinous silica hydrates. The
entrance of calcareous matters, gypsum, zinc-white, etc,
should also be guarded against.
The subject of water-glass is here treated very briefly,
partly because the various processes of stereochromy, even
with their latest improvements, are very little used in this
country, and partly because the preparations of water-
glass specially made for the use of painters may be trusted.
To this latter observation I might add the remark that the
problem of thoroughly examining a commercial water-
glass solution for strength, purity, and due proportion of
silica to alkali, is too complex to be undertaken except by
a trained chemist.
Lime-water is the name given to the solution in water
of slaked lime, called in chemical language hydrate of lime,
calcium hydrate, and calcium hydroxide. To prepare it,
quicklime, which has been made by burning (as it is com-
monly called) a pure marble, or, preferably, Iceland spar,
is slaked with distilled water. The calcium hydrate formed
is placed in a wide-mouth stoppered bottle, and covered
LIME-WATER 103
with several times its bulk of distilled water. The object
of this treatment is to dissolve soda and some other soluble
impurities, the major part of which will be removed when
the watery liquid in the bottle is decanted from the un-
dissolved excess of calcium hydrate which should then be
again covered with distilled water which has been recently
boiled. The stopper should be well ground and smeared
with vaseline. The bottle should be shaken at intervals
in order that the water may take up as much calcium
hydrate as it can dissolve. After all, this amount is very
small, not exceeding, at 15° C, 0*172 part by weight per
hundred measures of lime-water. Thus a gallon of lime-
water, saturated at about 60° F., could not contain more
than 120 grains of calcium hydrate, corresponding to
90 grains of pure lime or calcium oxide, CaO. In
ordinary practice such a perfectly saturated solution is
not attainable, while the most carefully prepared and
strongest solution is sure to become weakened each time
the stopper of the containing vessel is withdrawn by the
removal of some of the lime in solution in the form of
carbonate of lime. The clearest lime-water, from this
cause and from its action on glass, always appears turbid
after a time.
Although so dilute a solution, lime-water gives the most
marked reactions of an alkali : it turns red litmus paper
blue, embrowns yellow turmeric paper, and imparts a
crimson hue to colourless phenolphthalein paper. It acts
energetically upon many organic and some inorganic
pigments, owing to its alkaline or basic properties. The
ease with which the Hme in lime-water unites with car-
bonic acid, forming carbonate of lime ( = calcium car-
bonate), and the bearing of this action, and of other
properties of caustic lime upon the materials and
104 BARYTA.WATER
processes of painting are discussed in Chapters II. and
XXIII.
Baryta-water has its uses, but cannot replace lime-water
in fresco-painting. It is a solution of hydrate of baryta,
barium hydrate, barium hydroxide, for these names all
belong to the compound, in distilled water. The distilled
water used should have been recently boiled and then
cooled out of contact with the carbonic acid of the air.
The barium hydrate used may be purchased in the form
of colourless crystals having the formula BaOgHg + 8 aq.
These, if not sufficiently pure, may be washed with cold
distilled water, or recrystallized from boiling water, in
which they dissolve very abundantly. A saturated cold
solution is made by placing rather more than i ounce of
these crystals in a bottle containing a pint of distilled
water : the bottle should be almost full, the stopper should
be smeared with a little vaseline. If the crystals dissolve
completely, after repeated agitation, a few more should
be added so as to leave a small excess at the bottom of
the bottle. If the solution be clear it may be used directly
from the bottle, as required ; if filtration be needed, a
glass plate should be placed on the funnel during the
operation to prevent free access of air, and the clear filtrate
should be received at once in the bottle in which it is
to be preserved. A solution of barium hydrate saturated
at 15° C, contains nearly 2*9 grams of BaO in 100 cubic
centimetres, or 2,023 grains per gallon. It is thus about
seventeen times stronger than a solution of calcium hydrate
saturated at the same temperature. Baryta-water, as it
is called, is a powerfully alkaline liquid, becoming covered
with a film of white barium carbonate on exposure to the
air. By blowing air from the lungs through a glass tube
into baryta-water, a dense white precipitate is formed.
BARYTA. WATER 105
Unfortunately, the binding power of barium carbonate is
almost nil, so that baryta-water in mural painting is of
service, not directly as a medium, but for destroying
traces of calcium sulphate (gypsum) in the plaster-ground,
and thus liberating a corresponding amount of lime-water.
It may also be used for testing the effect of an alkaline
earth on the powdered pigments which it is proposed to
use in the work, in order to see if they can withstand its
action ; those unaffected by baryta will prove to be
unchanged by Hme.
CHAPTER X
SOLVENTS AND DILUENTS
The liquids to which attention is directed in the present
chapter are, with very few exceptions, not miscible with
water. Of water itself it is not necessary to say anything
beyond this, that distilled water is best adapted for almost
every purpose to which this liquid is applied in the prepara-
tion of pigments, as a solvent for gum, honey, etc., and in
the practice of painting in water-colours. Next to dis-
tilled water may be ranked rain-water collected in the
open country, then the softer kinds of water yielded by
some streams, springs, and wells. Waters containing
more than 20 or 30 grains per gallon of solid matters in
solution should be avoided as far as possible. It should
be noted that very hard waters tend to curdle or precipi-
tate the particles of colouring matter in the water-colour
paints which they may be used to dilute.
Before considering the chief solvents and diluents, a
list of the most important of those which have been ob-
tained in a pure state or isolated may be given. Most of
these are artificial or laboratory products, the natural
liquids employed in the processes of painting being mix-
tures, not infrequently both variable and complex. In
the foUov/ing table the several definite compounds in-
cluded are arranged according to their boiling-points, those
106
SOLVENTS
[07
which boil at low temperatures being placed first : an
asterisk indicates that the liquid is miscible with water :
TABLE
OF SOLVENTS
Name
Boiling-point
Specific
Gravity
Formula
Ether -
35°C.= 95° F.
- 0719 -
C.HioO.
Carbon bisulphide
46°
= 115°
- 1-271 -
cs,.
♦Acetone
56°
= 133°
- 0798 -
CsHeO.
Chloroform -
61°
= 142°
- I -500 -
CHCI3.
♦Wood-spirit -
66°
= 151°
- 0798 -
CH4O.
♦Alcohol
78°
= 172°
- 0794 -
C^HgO.
Benzene
81°
= 178°
- 0-884 -
CeHg.
Toluene
111°
= 232°
- 0869 -
C.Hg.
Epichlorhydrin
117°
= 243°
- 1-191 -
C3H5CIO.
Perchlorethylene -
121°
= 250°
- I -620 -
CXI,.
Amyl alcohol
131°
= 268°
- 0-814 -
C5H12O.
Pinene -
160°
= 311°
- 0-859 -
CioHjfi.
Cineol = eucalyptol
173°
= 343°
- 0930 -
CioH.sO.
Cymene
175°
= 347°
- 0-858 -
C10H14.
Sylvestrene -
176°
= 349°
- 0-851 -
CioHjg.
Limonene
177°
= 351°
- 0850 -
^10^16.
Dipentene
177''
= 351°
- 0846 -
CioHi6-
Citral -
228°
= 442°
- 0-897 -
C,oH,eO.
Geraniol
230°
= 446°
- 0-894 -
C.oHasO.
The pinene, sylvestrene, limonene, and dipentene named
in the above table are examples of what are now called
terpenes. Mixtures of these and of a few other terpenes
of less importance constitute what is generally known as
oil or spirit of turpentine. Terpenes are very frequent and
often very abundant constituents of the volatile, ethereal,
or essential oils extracted from plants. Some, however,
of these volatile and strong-smelling essences consist
mainly of liquids containing oxygen, such as cineol or
eucalyptol, C^oH^gO, which occurs to a large extent in the
oils distilled from many species of eucalyptus. Besides
the compounds in our list and a certain number of essen-
io8 ETHER
tial oils from plants, we shall have to consider some of the
more volatile liquid constituents of natural petroleum and
of artificial paraffin oils. The fixed or fatty oils, which
are constantly used in painting and in the manufacture of
varnishes, have been already discussed in Chapter V.
Ether, often called sulphuric ether, is a very mobile
liquid of extreme volatility, and possesses a penetrating
odour. Its vapour, given off" freely at ordinary tempera-
tures, forms with air a highly inflammable and explosive
mixture. Great care is therefore required in using this
liquid ; no light must on any account be brought near it.
It does not mix with water, but floats on the surface,
although it dissolves in water to the extent of about lo per
cent. Commercial ether contains water and alcohol along
with traces of other impurities. It is seldom necessary to
remove the alcohol from it (for varnish-making, etc.), but
it can be got rid of by repeatedly shaking the crude ether
with water, whereby much ether also is dissolved away.
The water present interferes seriously with the use of
ether as a solvent for resins, etc., but it may be removed
by careful rectification with fused calcium chloride, that
substance having previously been allowed to remain in
contact with the liquid for a day. A final distillation
from a little metallic sodium completes the drying of the
ether and also removes, if used in sufficient quantity, the
alcohol present. Great care is necessary in distilling
ether to secure, by a current of ice-cold water in the
condenser, the condensation of the vapour.
Carbon Bisulphide. — This heavy, oily but volatile liquid
readily gives off" vapour at ordinary temperatures. It is
poisonous, and the same care in manipulating it must be
taken as that insisted upon in the case of ether. The
smell of the ordinary commercial bisulphide is most offen-
ACETONE 109
sive, but it is now possible to purchase a specially purified
sort from which a particularly disagreeable sulphur-com-
pound of nauseous odour has been removed. Carbon
bisulphide sinks in water : it is a powerful solvent for
many resins, and mixes perfectly with the fixed and
essential oils in all proportions.
Acetone also occurs in crude wood-naphtha. It has a
penetrating but agreeable odour. It is miscible with
water, alcohol, oils, etc., and dissolves many resins,
camphor, fixed oils, and allied bodies. It is sometimes
serviceable as a solvent for discoloured varnishes on
pictures. Commercial acetone is very impure, containing
wood-spirit, empyreumatic oils, and water.
Chloroform is another powerful solvent of resins. It has
a pungent but sweet taste, is not miscible with v/ater, and
is very heavy. Commercial chloroform often contains
alcohol and other foreign matters, from most of which it
may be purified by redistillation from a little oil of vitriol
followed by a second distillation from fragments of quick-
lime. For making varnishes neither water nor alcohol
should be present in chloroform, but there are other
impurities which do not interfere with its employment
for such a purpose.
Wood-spirit, or methyl-alcohol, is a constituent of wood-
naphtha, a product of the destructive distillation of wood.
It rarely occurs in commerce in a state even approaching
to that of purity. It is miscible with water in all propor-
tions, but not with fixed oils. When free from water it
may be used as a solvent for some resins, and for remov-
ing discoloured varnish from oil-paintings. Methylated
spirit now contains in 100 measures 9^ measures of crude
wood-spirit and ^ measure of petroleum oil, the remainder
being rectified spirit of wine.
no ALCOHOL
Alcoholy or pure spirit of wine, is met with in commerce
practically free from all impurities save water. Proof
spirit, rectified spirits of wine, and methylated spirit,
though of service in cleaning oil-pictures and for many
other purposes, ought not to be used in the preparation of
varnishes. For this purpose pure alcohol, often called
absolute alcohol, is required ; but provided that it con-
tains no water the presence of wood-spirit is no drawback
to its use. In commerce, nearly absolute alcohol, made
both from spirits of wine and from methylated spirit, is
obtainable ; but it may be prepared by operating upon
the strongest available spirits of wine in the following
manner : The spirit is distilled in a water-bath until no
further strengthening of the alcoholic distillate is secured
by repetition of the process ; then a dry retort is half-
filled with small, clean, hard fragments of quicklime, the
strong spirit is poured upon these so as to somewhat
more than cover them, and then the whole is left over-
night ; distillation from a water-bath is then commenced,
when it will be found that a spirit comes over which con-
tains no more than one part of water in two hundred.
Even this small proportion may be removed by redistilling
the alcohol from a very little metallic sodium. The last
distillate, when a small portion of it is shaken up with its
own bulk of benzene, should mix perfectly with the latter,
causing no turbidity. But it should be borne in mind
that absolute alcohol is a very hygroscopic liquid, greedily
absorbing water from the air ; it must, therefore, be kept
in well-stoppered bottles, filled almost completely. In
absolute alcohol some of the more intractable resins, even
some kinds of copal, readily dissolve. The specific gravity
of absolute alcohol at 15^ C. is '794, while, if it contains
but I per cent, of water, its specific gravity is distinctly
higljer, namely, 797.
BENZENE III
Benzene is employed not only as a solvent, but as a
diluent of the medium or oil employed in painting. It is
obtained from the lighter naphtha separated in the frac-
tional distillation of coal-tar. The benzene (also called
benzol) of commerce is rarely pure. The presence of small
quantities of higher hydrocarbons of the same series is of
little moment, but it also contains about one half per cent,
of a sulphur compound called thiophene (C^H^S), to which
the offensive odour of ordinary benzene is partly due.
Thiophene is, however, much more soluble in cold oil of
vitriol than is benzene, and may be removed by several
treatments of the benzene with small quantities of this
powerful acid. Benzene thus purified can now be pur-
chased. Benzene is a mobile liquid, not miscible with
water, but dissolving readily in all proportions in most if
not all of the liquids now being described. It dissolves
oils and very many of the harder as well as all the softer
resins.
Toluene^ commercially known as toluol^ much resembles
benzene, and may be used for the same purposes, although
it is less volatile. Conunercial toluene has a disagreeable
smell, arising from the presence of a sulphur compound
(thiotolene), which is more difficult to remove from the
liquid than the thiophene from benzene.
Toluene of good quality and at a moderate price may
be obtained from Kahlbaum of Berlin. It constitutes a
useful diluent and solvent when used with the spirit-fresco
medium.
Epichlovhydyin has been employed as a solvent for some
of the copals and other intractable resins in the manu-
facture of varnishes. Without further trials it would be
unwise to employ this liquid in artistic painting, yet the
resins dissolved in it seem to retain much of their original
112 TERPEN ES
toughness and hardness. The same statement may be
made in reference to several other similar chlorine com-
pounds, which possess the merits of cheapness and non-
inflammability.
Amyl alcohol, the chief constituent of fusel-oil, is used
by picture-cleaners for the removal of discoloured varnish.
Its vapour is suffocating and even poisonous.
Pinene, Sylvestrene, Limonene, and Dipentene, with several
other similar compounds, are the main constituents of the
various liquids to which the ordinary name of turpentine,
or, rather, spirit or oil of turpentine, is applied. All these
liquids are hydrocarbons, having the same composition in
loo parts, expressed by the empirical formula Cj^H^g.
But these liquids — of which about ten are known — differ
from one another in some of their chemical and physical
characters, such as oxidizability, boiling-point, specific
gravity, and action on light. The extreme importance
of turpentine in the process of oil-painting, and in the
manufacture of varnishes, warrants a somewhat full con-
sideration of its several constituents.
Turpentine, properly so called, is not a liquid, but the
solid or semi-solid resinous secretion of many trees, chiefly
coniferous. Some exudes naturally, but much more is
obtained by artificial incisions. It consists of a mixture
of one or more true resins and resinous acids in which
oxygen is present, with one or more liquid hydrocarbons
which contain (as the name imports) nothing but carbon
and hydrogen, and therefore no oxygen. These hydro-
carbons are called in chemical language ferpenes, a term
by which they will be designated henceforth in the present
chapter. On distilling the crude turpentine or resins
alone or with water, or in a current of steam, the terpenes
distil over while the solid part remains behind ; this, on
TERPEN ES 113
fusion, is called rosin or colophony. It need not be
further considered, as it is of no value in painting, being
friable and more or less strongly coloured ; it is, however,
employed in making certain ' dryers,' known as resinates
(or better, rosinates), containing cobalt, manganese, etc.
We confine our attention, therefore, to the distillate or
terpenes. It should be added, however, that the leaves,
cones, and other parts of many coniferous trees, them-
selves yield various terpenes when submitted to distil-
lation, and that many of the volatile or essential oils of
aromatic plants other than conifers contain or consist of
terpenes. The oils expressed from the rinds of lemons
and oranges afford illustrations of this remark.
Terpenes differ from one another in several obvious and
in several obscure ways. Even now the chemistry of
these liquids is not by any means clearly and completely
unravelled. We need not here concern ourselves with
those minute differences in chemical and physical pro-
perties by which the identity of individual terpenes is
established, but may confine our attention to their most
salient characteristics. Of these none is more important
than the behaviour of terpenes with regard to atmospheric
oxygen. Some of these liquids absorb oxygen readily,
and to a large extent, from the air, becoming thereby
resinified — in fact, they thus yield sticky, resinous, semi-
solid bodies, closely resembling the crude turpentine from
which they have been prepared. Everyone who has had
occasion to use spirit of turpentine frequently must have
noticed the production of a sticky substance about the
neck of the bottle in which this liquid has been kept.
Moreover, the spirit of turpentine itself will often have
been noticed to have become cloudy, viscid, or almost
solid, especially if it has been contained in a bottle fre-
8
114
TERPENES
quently opened, and not quite full. Besides these obser-
vations another will have been made — different specimens
of spirit of turpentine will have been found to differ
much as to the rate at which these changes have taken
place. Some samples, even in half-full bottles, remain
clear and limpid for long ; others become thick, opaque,
and sticky in a few weeks. Such changes are undesirable
•in a solvent, diluent, or painting medium, on many
grounds. The resin formed is an unsatisfactory one —
soft, sticky, and contractile. The liquid decreases so
greatly in mobility, and increases so greatly in viscosity,
that its utility in thinning oil pigments, and in making
fine touches, is greatly impaired. And this thickening of
the liquid is accompanied by the production of acid sub-
stances and of water, which affect injuriously the ease of
working and the stability of the picture. Spirit of tur-
pentine should disappear by evaporation quickly and
completely from the painting into which it has been intro-
duced. Now, if it be easily oxidizable, even if it be kept
from experiencing change before it is actually employed,
it will, during the very time in which it is being used,
attract oxygen; so that though a great part of it will
escape by evaporation, the remainder will resinify on the
canvas itself, adding a sticky deposit to the drying oils
and hard resins which may have been used as the paint-
ing medium. It is clear, from all the above considera-
tions, that the greatest care ought to be taken in selecting,
in the first instance, such a sort of spirit of turpentine
as will resist oxidation under ordinary conditions. Even
an inferior spirit may be used, with a minimum of dis-
advantage, if immediately after distillation it be poured
into a number of small bottles, so as to fill each of them
completely ; they should be at once closed with sound
TERPENES
113
corks. In this way the contents of a bottle may be used
up very soon after it has been opened. Another precau-
tion may be taken : A few small fragments of hard quick-
lime may be placed in each bottle to absorb any moisture
produced by oxidation, and also the acid bodies which are
formed at the same time. Even with the choicer samples
of spirit of turpentine, which pass much less easily into
resins, this precaution is desirable ; but in this case the
employment of many small bottles is unnecessary, and
it will suffice to put a few hard pieces of lime, free from
powder, into a pint or quart bottle, and then to fill it
with the spirit. The clear liquid may be poured off as
required for use, any disintegrated particles of lime sink-
ing readily to the bottom of the vessel.
Before giving details as to the sources and character-
istics of the best terpenes, it may be useful to mention that
commercial samples of spirit of turpentine m^ay be tested
and compared by means of a very simple experiment.
Obtain the required number of small flat-bottomed, conical
glass flasks with wide mouths, one flask for each sample ;
these flasks are known as Erlenmeyer's. Into the flasks
pour enough of the several samples to cover the bottom
to the depth of one-eighth of an inch ; label each flask
to correspond with the sample, and lightly close each
mouth with a plug of carded cotton — the date of the
experiment should be added on the label. Shake each
flask so as to cause a number of bubbles to be formed in
the liquid ; the more rapidly these bubbles break, the
better is the sample. Repeat the experiment of shaking
the samples at short intervals for a few weeks — notable
changes in the viscosity of the oils will be observed
sooner or later. Any sample which after one month
remains clear, and in which the bubbles formed on agita-
ii6 TERPEN ES
tion break almost as quickly as at first, may be accepted
as of good quality. Another test for discriminating
between the samples, so far as their state at the time of
the experiment is concerned, is the very simple one of
placing one drop of each oil upon a sheet of writing-
paper, and gently warming the translucent stain it forms ;
with a good oil the mark completely disappears.
Two other obvious characteristics of different samples
of spirit of turpentine may now be noticed — namely,
odour and boiling-point. Some samples have a much more
agreeable scent than others ; the vapour of these seems
to have a less marked tendency to produce headache
than that of the pungent and cruder-smelling varieties.
The range in boiling-point is not very extensive ; but
it may be taken as about 25° C, the figures ranging
from 155' to i8i.° Samples having lower boiling-points
evaporate more quickly than those which enter into
ebullition at higher temperatures. The solvent power
on resins differs with different kinds ; this is a property
which is of importance in varnish-making, but very little
accurate knowledge exists on this point. But there is one
characteristic almost peculiar to the terpenes which must
not be overlooked. When oxidizing they possess, unlike
the alcohols and benzene and petroleum spirit, a power of
starting or increasing the absorption of oxygen by linseed
or other drying oils ; in fact, they act as siccatives. This
property is constantly utilized in oil-painting ; probably it
is connected with the formation of hydrogen peroxide which
passes on its surplus atom of oxygen to the oil present.
A few of the more important turpentine oils may now
be named :
American oil, chiefly from Finns palustris and P.
Tceda.
TERPEN ES 117
Austrian oil, partly from Pinus Laricio, partly from
P. Pumilio.
Burmese oil, from Pinus Khasya.
French oil, from Pinus Pinaster ( = P. marifima).
German oil, from Pinus sylvestris, P. Cemhva, P. Abies,
P. vulgaris, etc.
Juniper oil, from Juniperus communis.
Russian and Swedish oils, chiefly horn Pinus sylvestris
and P. Ledebourii.
Strasburg oil, from Abies pedinata.
The above-named turpentine oils are accompanied by
small amounts of various resins, camphor, and other
oxygenated bodies, from which they may be separated
by treatment with caustic potash, metallic sodium, and
fractional distillation.
From the above-named liquids a number of terpenes
have been isolated. Among the better known of these
the following may be mentioned.
1. Pinene, with a boiling-point of 160° C. It exists in
two forms, distinguished by their rotatory power on
polarized light : dextro-pinene is the chief constituent
of German and American oil of turpentine. The other
form of pinene, laevo-pinene, occurs in large proportion in
French oil of turpentine ; it absorbs oxygen less readily
than dextro-pinene, and therefore resinifies more slowly.
In consequence, it is more suitable for artistic uses, such
as the thinning of paints, or as an ingredient of varnishes.
2. Phellandrene. — Boiling-point 171° to 172°. This
terpene has been separated from eucalyptus oil, that is,
from the oil obtained by the distillation with water of
the leaves of one of the numerous species of eucalyptus,
E. amygdalina. It is one of the most alterable of all
terpenes, and the oils containing it should be avoided.
ti8 TERPENES
3. Limone7te.-~ This terpene, like most of the others,
occurs in two forms or varieties, having opposite actions
on polarized light. It is sometimes called citrene. It
boils at lyy*'. It is best prepared from orange-peel oil,
which yields over 90 per cent, of dextro-limonene when
distilled from caustic potash. When pure it is less alter-
able than dextro-pinene and phellandrene, though it
resinifies after a time.
4. Sylvestrene. — Boiling-point 176°. When pure it has
the smell of bergamot, but generally presents the odour
of fir-wood. It is dextro-rotatory, and forms the chief
constituent of Russian and Swedish oil of turpentine,
and of some of the German oils. On exposure to air
it oxidizes readily, and leaves a sticky resin. It has
a particularly strong odour.
5. Dipentene, which boils at about 177° C, is optically
inactive, and may be made by heating some of the other
terpenes to 250° — 270° for some hours, or by mixing
dextro- and Isevo-limonene together. The only natural
product in which dipentene is known to occur is the
volatile oil which accompanies common camphor. Its
odour resembles that of citron oil : it resinifies to about
the same extent as limonene.
From the preceding descriptions it may be gathered
that of all the above terpenes, limonene and dipentene are
the least alterable. They have, however, somewhat high
boiling-points, and evaporate more slowly than most of
the other terpenes.
Here it may be mentioned that the presence of water
in a terpene, or a mixed essential oil, may be detected by
the cloudiness which it shows when mixed with thrice
its volume of benzine or of petroleum-spirit. To remove
traces of water from any of the less volatile liquids we
TERPENES
19
have been considering, without having recourse to distil-
lation from caustic potash, or from quicklime, the follow-
ing simple procedure may be adopted : A glass flask is
three-fourths filled with the liquid, and then it is kept at
a temperature of 110° to 120° C, so that the moisture
present is disengaged as vapour without the terpene or
essential oil itself boiling : drops of moisture will condense
in the neck of the flask, and may be removed from time to
time by means of a roll of blotting-paper. The mouth of
the flask should be loosely plugged with carded cotton. Of
course this process is applicable only to liquids which boil
at temperatures considerably over 120°, like the terpenes.
In connexion with the terpenes two other liquids and
one solid remain to be mentioned. The liquids are * oil of
amber' and 'oil of copal.' These are obtained by strongly
heating the resins in question. They are employed as
efficient solvents for the harder resins. Oil of amber may
be obtained in commerce at a moderate price. Its offen-
sive smell, partly due to compounds of sulphur, may be
lessened by adding to it some white lead and solid caustic
potash, and afterwards distilUng it. It contains amongst
other liquid constituents at least one terpene. Its boiling-
point rises, as distillation proceeds, from 110° C. to 260°.
Camphor is expressed by the formula C^gH^gO, and is
obtained chiefly from Cinnamomum Camphora, a tree of
Formosa, China and Japan. It is a tough crystalUne
solid of penetrating odour and pungent taste. It is
soluble in all the liquids named in the present chapter.
Although it boils at so high a temperature as 204° C, it
readily and rapidly volatilizes at ordinary temperatures.
It is used to aid the solution of some of the harder resins
in the making of varnishes, but its presence in a varnish
is objectionable, for it slowly escapes after the apparen-
I20 PETROLEUM-SPIRIT
drying-up of the varnish, and thus causes a deterioration
of the lustre and continuity of the resinous film.
Petroleum -spirit. — When native petroleum and the
similar materials obtained in the distillation of bitumin-
ous shales, etc., are submitted to fractional distillation,
the more volatile portions which come over first con-
stitute the liquids variously known as benzine, gasoline,
benzoline, ligroine, petroleum-naphtha, petroleum-ether,
and petroleum-spirit. This liquid consists entirely of
hydrocarbons, some of which belong to the paraffin series,
while others are naphthenes. Their boiling-points are all
under 170° C, while some of them boil as low as 50° ;
indeed, commercial samples of petroleum-spirit often
begin to enter into ebullition at a lower temperature even
than this. The series of petroleum products may be
roughly grouped thus :
Petroleum-spirit boils below 170° C. ; specific gravity,
•6 to 7.
Lamp-oil, kerosene, photogen, or paraffin-oil, boils
between 180° and 220°, and has a specific gravity of 78
to -82.
Solar-oil, lubricating-oil, vaseline, and paraffin-wax, are
heavier products, with a range of specific gravity from
•83 up to '94. Their viscosity increases with their density
until the semi-solid vaseline and the solid paraffin-waxes
are reached. The latter substances have been described
already, the former are not available in painting : in fact,
their presence even in traces in petroleum-spirit — an
extremely useful solvent and diluent— should be carefully
guarded against. They neither escape by evaporation
nor harden in the lapse of time. Thus petroleum-spirit
remains alone for further consideration.
As a solvent for resins, and as an extremely volatile
PETROLEUM -SPIRIT 121
and very thin liquid for diluting oily vehicles and paints
in the process of oil-painting, the variety of petroleum-
spirit which boils between 50° and 70° C. is the most
suitable. It contains hydrocarbons represented by the
formulae CgHjg and CgH^^. It must be used with great
caution on account of its easy inflammability and the
readiness with which it gives off a vapour, which, when
mingled with atmospheric air, is highly explosive. It
may be used for many purposes in lieu of benzene (from
coal-tar naphtha), being much cheaper and quite as
efficient. A drop of this variety of petroleum - spirit
on paper evaporates very quickly, leaving no greasy
stain.
Another variety of this petroleum-spirit is obtained
by collecting apart the fractions which boil between
100° C. and 130°. These contain heptane (C^H^g), octane
(CgHjg), heptylene (C^H^J, and octylene (CgH^g), and
other hydrocarbons. This mixture is less volatile than
that just described, it dries more slowly, and is a less
energetic solvent.
A third variety boils between 130° and 170° and is avail-
able for many of the purposes for which turpentine-oil is
employed. It is not advisable, in my opinion, to use
fractions having a higher boiling-point than 170° C. as
additions to the pigments and vehicles of oil-painting,
for, though their slow drying is sometimes an advantage,
there exists the danger of their incomplete evaporation
from the painted surface. If they remain even in traces
in the finished work after it has been varnished, they may
give rise to the same accidents as are caused by the
treacherous though seductive asphaltum.
It should be remembered that the various petroleum
liquids just described do not resinify, nor do they leave
122 OIL OF SPIKE LAVENDER
any permanent stain or mark upon paper which has been
moistened with them.
In the Table of Solvents on p. 107 three liquids contain-
ing oxygen and related to the terpenes find a place. Two
of these, cineol or eucalyptol, and geraniol, represented by
the empiric formula C^^H^gO, are alcoholic in consti-
tution ; while the third, known as citval, belongs to the
aldehydes. These and several other allied oxidized com-
pounds, as well as a few ethereal salts known as esters^
enter largely into the composition of certain essential oils
occasionally used in oil-painting. We will introduce a
few remarks concerning some of these compounds under
the names of the essential oils of which they are important
components.
Oil of Spike Lavender is obtained by distillation from the
flowers of a species of lavender, Lavandula spica. Its
specific gravity varies from -905 to "918 : it dissolves in
three volumes of 70 per cent, alcohol. It contains about
30 to 40 per cent, of an alcohol, linalol (C^QHigO), about
5 per cent, of an ester, linalyl acetate, a considerable
quantity of cineol (eucalyptol), and a small proportion
of terpenes. It is often adulterated with rosemary oil.
When spike oil is exposed for a long time to the air, the
part which does not evaporate gradually thickens, owing
to the oxidation of its constituents. This thickened oil,
although usefully employed in the application to porcelain
of enamel colours, is useless in oil and spirit-fresco
painting. To preserve spike oil in its thin and mobile
state, the precautions recommended in the case of tur-
pentine (p. 114) should be taken. Spike oil is a powerful
solvent of resins ; it is one of the components of Gambier-
Parry's spirit-fresco medium.
Oils of Eucalyptus are derived from many species of
OIL OF ORANGE 123
eucalyptus, and differ from one another widely as to the
relative proportions in which their components exist, and
also in their odour. Cineol, or eucalyptol, is the chief
and most characteristic constituent of the majority of
them, occurring to the extent of 50 to 65 per cent, in the
oil from E. globulus, the commonest of all kinds. On the
other hand, the oil from E. maculata^ var. citriodora, some-
times contains no less than 95 per cent, of citronellal, an
alcohol. This oil is of particularly agreeable odour, as
are also the oils of E. cneorifolia, E. corymhosa, and E,
dealhata. For a diluent in oil-painting it is probable that
any one of these oils may be chosen with equal propriety,
our choice being guided by the odour of the sample.
Oil of Lemon. — Though the terpenes known as dextro-
and laevo-limonene constitute the main portion of this oil,
yet its characteristic odour is partly due to the presence
of 6 to 9 per cent, of the aldehyde citral mentioned above.
This compound is occasionally separated from lemon-oil ;
by its removal the remainder becomes richer in terpenes
and more fitted for the use of the painter.
Oil of Orange. — This essential oil, which, like that of
lemon, is obtained by expressing the rind of the fruit, is
remarkably rich in limonene (dextro-limonene). This,
with small quantities of other terpenes, makes up over
95 per cent, of the oil. Although the expressed oil pos-
sesses a yellow colour, it may be obtained absolutely free
from colour and of agreeable odour, by distillation under
reduced pressure.
Oil of Rosemary. — This oil varies in specific gravity
between -9 and '918. It contains about 20 per cent, of
an ester, bornyl acetate (C^HgoOgi), and 6 per cent, of
borneol. Its other constituents include lineol and two
terpenes.
124 CYMENE
Cymene. — There are twenty or more varieties of the
hydrocarbon expressed by the empirical formula C^qH^^.
The best known of these is the agreeable lemon-scented
liquid contained in cumin oil from the seeds of Cuniiniim
cyminum. Its full scientific name is paramethylisopropyl-
henzene, while its constitution is shown by the formula
CH3.C6H4.CH(CH3)2. This hydrocarbon serves the
same purpose as a diluent as the terpenes, but is too
costly for general use. Doubtless some of the other
forms of the C^qHj^^ hydrocarbon might be similarly em-
ployed. The range of their boiling-points lies between
1 68° and 204° C. Some are solid at ordinary temperatures.
CHAPTER XI
SICCATIVES OR DRYERS
The terms ' siccatives ' and ' dryers ' are applied to three
classes of substances. Perhaps the most correct or appro-
priate application of these words is to those metallic com-
pounds which are used in order to increase the rate at
which the drying oils harden, but in the literature of the
subject we often find that drying oils which have been
thus treated, and likewise certain resinous solutions, are
spoken of as siccatives. In the present chapter we describe
the dryers proper only, referring our readers to the chapters
on oils and on varnishes for the necessary particulars con-
cerning the other materials which may be included in the
group under discussion.
Lead and several of its salts have been long and widely
used as dryers. Metallic lead in the form of foil, litharge
or lead protoxide, minium or red lead, lead peroxide, sugar
of lead or lead acetate, the basic lead acetate, and white
lead itself, have all been used in this way, chiefly for the
purpose of making linseed or other painting oil dry more
quickly. Some of these compounds, particularly sugar
of lead, have been introduced into the very picture itself.
It was a common practice to employ powdered sugar of
lead or a solution of this salt in water to hasten the drying
of vehicles and of slow-drying pigments which have been
125
126 MANGANESE DRYERS
ground in oil. I have seen one of the results of this com-
mingling of sugar of lead with the medium or the paint
in the production of an immense number of small spots
in the picture, sometimes appearing through the surface-
varnish in the form of a white efflorescence. This
efflorescence consists at first of lead acetate in crystals,
but these soon attract carbonic acid from the air and
become lead carbonate, which, in its turn, is changed
into lead sulphide by the action of sulphuretted hydro-
gen. This tendency of the lead compounds to yield
brown or black lead sulphide is, indeed, the great draw-
back to any use of these substances as dryers. When
oil is left in contact with them, and especially when heat
is applied to the mixture, some of the lead dissolves,
forming, with the fatty acids of the oil, lead-soaps. These
soaps are distributed uniformly throughout the oil, and
help to make it dry and harden quickly. The same action
occurs when white lead is ground as a paint with oil, and
has been urged as an objection to the use of those white
leads which contain hydrate of lead, a compound which
acts upon oil more quickly and thoroughly than the car-
bonate of lead.
It will be seen, however, that while there may be
reasons for permitting the use of a single lead pigment
which possesses this peculiar property, there can be none
for introducing into every part of a picture oils or other
materials which contain a metal, like lead, so liable to
cause discoloration and darkening, when other and per-
fectly innocuous substances are available for producing
the same siccative effects. On this account we omit
further reference to the lead compounds, which have been
and are still employed in the preparation of strongly-
drying oils, etc., but pass on to the
MANGANESE DRYERS 127
Manganese compounds, of which the dioxide, the
hydrated protoxide and sesquioxide, the borate, the
oxalate, and the Hnoleate are the most important.
Manganese dioxide, the black oxide, MnOg, is used in
the form of a powder obtained by grinding the mineral
pyrolusite. As the effectiveness of this compound is made
complete only by the use of oil of vitriol, which needs
subsequent neutralization with lime, it cannot be recom-
mended as a material for rendering linseed oil intended
for painting, or for making picture-varnish, more drying.
The difficulty of preparing the manganese hydrates above
mentioned constitutes an objection to their employment
for this purpose. But the borate, the Hnoleate, and the
oxalate of manganese may be obtained in commerce in a
state of sufficient purity for our present purpose, and it is
to them that we wish to direct attention. Borate of
manganese may, moreover, be so easily prepared, that it
is worth while to give here the necessary directions.
One pound of pure manganese sulphate is dissolved in
six pints of distilled water, the solution being filtered if
cloudy. A few drops of the liquid are now to be tested
with caustic soda solution — the precipitate formed should
be white ; if it show a greenish, yellowish, or greyish hue,
iron is probably present, and it will be necessary to treat
the whole of the solution with caustic soda until a white
precipitate falls, and then to filter it again. In order to
produce manganese borate, a boiling saturated solution
of pure borax is added to the manganese sulphate solu-
tion until no more precipitate falls. The precipitate is
collected on a filter and washed with hot distilled water
until the wash-waters show no turbidity when a solution
of barium chloride and a few drops of dilute hydrochloric
acid are added to the last portion coming through the
128 DRYERS
paper. The borate of manganese is then dried in a warm
place, and finally in the water-oven. One grain of it,
warmed with linseed oil, is sufficient to render an ounce
of the latter highly drying (see Chapter V.). The oxalate
or the linoleate of manganese may be used in the same
way, and there are also met with certain manganese
* resinates ' which may be employed similarly. Cobalt
resinate is also a powerful dryer.
^ The quantities of lead or manganese compounds
necessary to confer the drying character on oil are very
small. With -2 to i 'o per cent, of the weight of the oil
the maximum effect is so nearly attained that any further
addition is unnecessary. If the manganese in the form
of resinate or linoleate be dissolved in ten times its weight
of turpentine, a siccative having very little colour but
possessed of great power is obtained ; this, when added
to the oil in the proportion of from i to 20, i to 50, or even
I to 100, confers the drying character upon it without
any further heating. Moreover, if this oil be allowed,
under carefully adjusted conditions, to absorb enough
additional oxygen for it to enter on the second period
(p. 53), it dries with great rapidity. Oil of this kind, pre-
pared with the smallest possible amount of a manganese
salt, is much to be commended for the use of the artist,
since it assures to the finished painting a longer life than
any other preparation of this kind.
Borate of lime and borate of zinc have been employed
not only for rendering oils more quickly drying, but also
in admixture with some of those oil-paints which dry with
difficulty. Being colourless they are well adapted for
use with white pigments, such as oxide of zinc. Several
of the siccative materials sold under various fancy names
consist of mixtures of these borates with carbonate of
DRYERS 129
zinc or oxide of zinc, manganese compounds being also
sometimes added. Another dryer in common use is white
vitriol or sulphate of zinc. Its siccative character is very
slight. Most of the other siccatives employed by artists
owe their efficacy to lead, or are resinous preparations.
Siccatif de Courtrai is a very dangerous mixture, heavily
loaded with compounds of lead ; Siccatif de Haarlem is
a resinous preparation, which produces, on drying, a hard,
brilliant, and tough film. This acquires, in course of
time, a deep yellow hue, which, however, hardly affects
the colour of the paints with which the siccative has been
employed, because of the small proportion used.
IF Here it may be mentioned that spirit of turpentine
may be regarded as a fairly efficacious dryer, especially
if the picture in which it has been employed is exposed
to sunlight. This property of turpentine (terpenes) is
not shared by the hydrocarbons of petroleum and of coal-
naphtha, such as petroleum spirit, benzene, and toluene.
This difference in chemical activity must be borne in
mind if we would correctly estimate and foresee the
results of employing these several liquids as vehicles or
diluents.
It may be well to remark in this place that many of
the volatile solvents, described in this chapter, are danger-
ously inflammable, and some are of a poisonous character.
CHAPTER XII
VARNISHES AND VEHICLES
When an oil, such as linseed, walnut, or poppy, has been
purified and made more quickly drying by one or other of
the methods already described, it is often called 'varnish.'
It has acquired the property of rapidly solidifying, when
spread as a thin layer, into a tough transparent substance,
endowed with a considerable degree of cohesiveness and
elasticity, yet rather soft withal. Now oil of this character,
although it has many uses in painting, is not quite hard
enough for some of the purposes for which a true varnish
may be required, but its defects may be amended by
associating with it one or more of the resins described in
Chapter VI. One class of varnishes is compounded in
this manner of two materials, oil and resin, both of which
are fixed or non-volatile. A second group of varnishes
consists of a resin dissolved in a volatile solvent. And
there are also mixed varnishes which contain at least three
ingredients — namely, a drying oil, a volatile solvent, and a
resin. As the varnishes which consist wholly of oil and
resin are thick and intractable, it is usual to thin these
according to the purpose for which they are intended, with
varying amounts of some volatile liquid or solvent, spirit
of turpentine being most frequently thus employed. In
order to avoid too elaborate a classification, it will be
130
MASTIC VARNISH 131
advisable to describe those varnishes which contain oil as
oil or fat varnishes, and those which consist wholly of a
resin and a volatile solvent as spirit varnishes. We de-
scribe the latter first, as their manufacture is easier and
their constitution simpler. In order to avoid repeated
references to the descriptions already given of the several
materials employed in making varnishes, it will be con-
venient to state once for all that the oils used are described
in Chapter V., the resins in Chapter VI., and the solvents
in Chapter X.
Mastic Varnish. — This is usually prepared by dissolving
mastic in spirit of turpentine, although other volatile oils
and even absolute alcohol may be employed. In order
to prevent the mastic from agglutinating together, warm
powdered glass, or warm fine white quartz sand, may be
added to the resin before it is mixed with the solvent. The
spirit of turpentine should be absolutely free from mois-
ture, the mastic may be in tears, or, preferably, have been
purified and dried as before directed. The materials are
introduced into a capacious glass flask fitted with a cork,
tube and condenser so arranged that, when the flask is
heated in a water-bath, the vapours given off" from the
solvent may be condensed and return to the vessel. The
temperature of the water-bath may be 100° C. if oil of
turpentine be used, but should not be allowed to rise
beyond 78° C. if absolute alcohol or 96 per cent, alcohol
(specific gravity '806) be substituted for the oil of turpen-
tine. The following receipt gives a varnish which con-
tains nearly 25 per cent, of its weight of mastic, but the
proportion may easily be increased or diminished :
14 ounces of mastic,
44 „ „ spirit of turpentine,
6 ,, ,, powdered glass, or fine sand.
132 MASTIC VARNISH
When the mastic has dissolved the varnish is allowed to
cool, and then poured off into a closed glass vessel, in
which it is allowed to rest until perfectly clear. Or it may
be clarified by filtration through a plug of dried carded
cotton fitted into a funnel. The funnel should be closely
covered with a ground-glass plate, but a specially con-
trived filtering apparatus has been designed for the pur-
pose of preventing any escape of vapour during the pro-
cess of filtration.
The varnish prepared according to this receipt is nearly
colourless, and leaves a brilliant glassy film when it evapo-
rates on a smooth surface. But this film is very brittle, and
easily abraded by gentle friction even with the finger, in
fact it consists of little more than the original mastic resin,
the fragility of which is well known. To obviate this
brittleness many plans have been devised. Sometimes
Venice turpentine, Canada balsam, or Elemi resin is
introduced in small quantity, not exceeding one-seventh
in weight of the mastic used. In consequence of such
admixture of a natural soft turpentine the varnish pro-
duced dries more slowly, and leaves a less brittle, tougher,
more adhesive, and more elastic film on evaporation.
Ultimately, however, these balsams become brittle like
mastic itself. This remedy is, therefore, of a temporary
character, but, at the same time, these additions do not
interfere with the ease with which the varnish, when old
and discoloured, can be removed from a painting by means
of solvents or of friction, without injuring the glazing pig-
ments which may lie immediately below it : they also
render the varnish more easy of application. The other
classes of substances added to toughen the resinous film
left by the drying of a spirit varnish, are fixed oils, and
those liquid paraffins which boil at temperatures above
COPAL VARNISH 133
170° C. A very small proportion of * manganese ' linseed
oil is, perhaps, the more effective and safer toughener of
the two, but its introduction involves the disadvantage
just named. In many French mastic varnishes camphor
is introduced for the same purpose to the extent of 5 to
8 parts for each 100 of mastic. The camphor, however,
gradually escapes by volatilization, the varnish losing its
fine lustre and becoming brittle and fissured. It should
be mentioned here that the more easily resinified varieties
of oil of turpentine, when used as solvents for mastic,
also toughen the resinous film left on the drying up of the
varnish, although the effect is not permanent. If alcohol,
benzene, light petroleum ether, or other non-oxidizable
solvents be substituted for any kind of essence of turpen-
tine in making mastic varnish, there is no doubt that the
brilliant films they yield are more brittle and less adhesive.
Sandarac and the various kinds of soft pale dammar may
be substituted wholly or in part for the mastic mentioned
in the receipt for spirit varnish above given. But if these
dammars be used great care must be taken that they are
themselves free from moisture, and that the oil of turpen-
tine or other solvent be also perfectly dry. It has been
recommended to employ oil of spike lavender instead of
oil of turpentine in making mastic varnish. The spike
oil in this case must be free from water, and freshly dis-
tilled : mastic varnish thus prepared has less tendency to
* bloom ' than the ordinary kind, but if pictures are var-
nished in a perfectly dry atmosphere and kept therein
till the surface has hardened, the formation of bloom is
minimized if not prevented.
A copal spirit varnish may be made by the use of
acetone, or of ether (both water-free), or of absolute
alcohol, light petroleum-ether, or benzene. The copal to
134 COPAL VARNISH
be dissolved may be either Sierra Leone copal, Zanzibar
copal, or Demerara copal, the first two yielding the
harder varnish, but the last-named being easier of solu-
tion, or, rather, dissolving less incompletely. The pow-
dered copal, prepared as directed previously (by exposure
to the air, and heating), or first fused, or at least heated
till it has lost from lo to 20 per cent, of its weight, is kept
in contact with four times its bulk of the solvent until it
is nearly dissolved. Three measures of dry oil of turpen-
tine are then added, and the mixture submitted to distil-
lation from a water-bath until three measures of the
acetone or other original solvent have been drawn over :
an efficient condenser must be used. If it be desired to
prepare a mixed varnish (partly oil or fat varnish),
I measure of 'manganese ' oil, and 2 measures of oil of
turpentine may be used in lieu of the quantity of turpen-
tine above mentioned, the distillation being then proceeded
with as before.
In another method of preparing copal (and amber) spirit
varnishes the resins duly prepared and powdered are
heated with the selected solvent under pressure — that is, at
a temperature above that at which the particular solvent
used boils under ordinary conditions. With purified oil
of amber, oil of copal, oil of turpentine, oil of spike, or
the heavier petroleum spirit, and on a small scale, glass
tubes hermetically sealed and heated to 200° C. may be
used, but if a higher temperature or more volatile solvents
be employed, copper tubes with screw stoppers are
necessary. But operations of this order can be carried out
safely and successfully only in a well-equipped laboratory
or factory by skilled operators, and it is therefore unneces-
sary to furnish further particulars in a work like the
present.
COPAL VARNISH 135
The preparation of fat or oil varnishes with the harder
resins is generally attended with considerable difficulty;
but there is as we have already mentioned, one way in
which the difficulty may be lessened. By the aid of one of
the powerful and very volatile solvents previously named,
we prepare a spirit copal or amber varnish ; we then add
the required amount of * manganese ' oil and draw off the
volatile solvent by distillation, thinning the resinous solu-
tion obtained with so much oil of turpentine as is necessary.
If the copal or amber employed has been first roasted or
fused, the varnish produced will be more or less dark in
tint ; it is on this account that the exposure of the powdered
resin to the air in a flat porcelain dish for seventy-two
hours, at a temperature (of 220° C.) which does not cause
discoloration, is recommended. But if, on the other hand,
the copal or amber be merely powdered, some part of it,
and that a considerable part, will probably remain undis-
solved though swollen, and will therefore be wasted. The
following process, in the main identical with one recom-
mended in the American edition of Mr. Erwin Andres'
work on varnishes, yields a pale and durable varnish when
Sierra Leone copal or other hard copal is employed, and
is doubtless well adapted for the preparation of amber
varnish also. As will be seen, it is based upon the pre-
liminary partial solution of the hard resin in chloroform,
or in light petroleum spirit of about the same boiling-
point. It may be stated at once that the proportions of
the five ingredients used are approximately 10 parts by
v/eight of copal or other hard resin ; 5 parts by weight
of dried powdered glass or sand ; enough chloroform to
cover the above substances ; 35 parts by weight of oil of
turpentine, and 10 parts by weight of ' manganese' oil.
The following is an outline only of the process. The
136 COPAL VARNISH
copal, after having been powdered and heated to 220° C.
for seventy-two hours, is mixed with the glass or sand,
and introduced into a retort ; chloroform in quantity
sufficient to cover the mixture is added. After the lapse
of twenty-four hours the dry oil of turpentine is poured
in, and an upright condenser is attached to the retort.
The retort is then heated to 50° or 60° C. for two hours,
so that the chloroform continually returns to the mixture.
Then the contents of the retort are allowed to cool, and
the condenser slanted downwards to allow of the chloro-
form being distilled over. This removal of the chloroform
having been effected at a temperature so low that very
little turpentine has come over, the remaining mixture in
the retort is heated once more with the condenser in an
upright position. The heat used must suffice to bring
the oil of turpentine into vigorous ebullition — in an hour
the whole of the copal should have dissolved. The ' man-
ganese ' oil wanted should now be heated to 100° C, and
then the copal mixture, when it has cooled to 70° C,
added little by little to it with constant stirring, the tem-
perature of the oil being maintained at 90° to 100°. When
the mixture is complete the source of heat is withdrawn,
but the varnish is still stirred for twenty minutes. Then
it is allowed to settle, until quite clear, in glass bottles,
or, if an appropriate filtering apparatus is available, it is
filtered. In the latter case a little hot oil of turpentine
may be used to extract any copal solution which may
remain with the powdered glass or the sand in the retort.
The older plan of preparing oil or fat varnishes with
hard resins is still that usually adopted ; but it yields
products which are darker in colour than those obtained
by the method just described, as the copal or amber used
has been previously heated or even fused, whereby it has
COPAL VARNISH 137
lost one quarter of its weight. One way of carrying out
this plan consists in melting the copal in one vessel, and
heating the oil until it commences to give off small bubbles
in another ; then half the oil is poured in a very thin
stream into the melted resin, and incorporated therewith
by constant stirring. Complete union having been effected
between the two materials, the mixture is incorporated
with the remainder of the hot linseed oil, any portions
adhering to the vessel being afterwards dissolved by means
of oil of turpentine ; 30 parts of melted copal, 100 parts
of linseed oil, and 70 parts of oil of turpentine, are pro-
portions often employed in carrying out the process we
are describing. This process may now be completed by
adding to the solution of copal in linseed oil J of a part
of manganese borate, stirring continually, and heating for
two hours, or until the solution has acquired the character
of a thick gold-coloured syrup which can be drawn out
into threads. This point having been reached, the heating
is discontinued, and the contents of the boiler allowed to
cool to 60° or 70° C, and then is added the warm oil of
turpentine which has been used to dissolve out any of the
copal solution clinging to the vessel in which that resin
was melted. Finally, the remainder of the oil of turpen-
tine is very gradually introduced with constant stirring.
Copal varnish prepared in the above manner ought to
dry in twelve hours or sooner. It is scarcely necessary
to say that this method of preparing varnish with copal
or other hard resin is one that no inexperienced person
should attempt ; not only is there some chance of partial
or total failure, but there is serious risk of fire. An easier
and less dangerous process requires a specially constructed
heater, which is kept hot by a water-bath. Melted copal,
copal or amber oil, * manganese ' oil, and oil of turpentine,
138 COPAL VARNISH
are the materials used. They are all introduced together,
and, as the temperature during the process of cohobation
does not exceed ioo°C., the time required is greater than
in the previously described process.
A good copal or amber varnish ought to leave a film
(on a sheet of glass) which combines the qualities of hard-
ness and toughness. The toughness is given by the oil,
the hardness by the resin. Such a film should not become
fissured even when it has been exposed to sunshine
during a year. Much of the copal varnish of commerce
is not made from true copal or anime at all, kowdi or
kauri resin (from Dammava atcstvalis), which is much
easier to dissolve, being employed instead — the product,
however, is decidedly inferior. Sometimes several resins
are mixed together in the preparation of a so called copal
varnish. A guarantee of genuineness, in which the name
or names and proportions of the resin or resins employed
is inserted, should always be demanded when buying
copal varnish. This ought to be furnished by the varnish-
maker himself, for artists' colourmen rarely prepare oil-
varnishes themselves.
An ingenious process for rendering hard copals soluble
in oil without roasting them in the ordinary way has
been recently devised and patented by H. Terrisse. It is
based upon the fact that the solid hydrocarbon, naphtha-
lene, a by-product in the manufacture of coal-gas, shares
with some other organic bodies, liquids as well as soUds,
the property of dissolving hard resins when heated with
them under pressure at a temperature not exceeding
300° C. In this way the greater part of the loss in-
curred when copals are roasted in the usual manner is
avoided, while the discoloration of the torrefied resin does
not occur. A mixture of i part of copal with 3 parts
of naphthalene is taken, and then heated for an hour or
MEDIUMS FOR OIL-PAINTING 139
two until complete solution has been effected. The mass
is then transferred to a suitable still, when the naphtha-
lene, being volatile, along with a little moisture and some
oil of copal separated in the operation, are distilled off.
Subsequent operations are two — the incorporation, under
pressure, with the altered copal of the necessary propor-
tion of linseed oil ; and, secondly, the addition of spirit
of turpentine to thin the product, which is pale in colour
and yields a sound varnish. Hard copals treated by this
process suffer changes less profound than those brought
about by the higher temperature involved in roasting
them ; they consequently retain more of their original
qualities of hardness and toughness when finally turned
into varnishes.
For the general use of painters in oil nothing more is
wanted than true copal or amber oil-varnish, a drying oil,
and a diluent. Of these three liquids a mixed medium in
general use is compounded by taking equal measures of
the three — varnish, oil, spirit of turpentine — and mixing
them together in small quantities as required. But con-
sidering the large quantity of oil already associated with
oil-pigments and present in copal or amber oil-varnish,
one-third of oil in the medium seems a somewhat high
proportion. I have proved by numberless experiments
that it may be reduced with perfect safety to the perma-
nence of the picture, although the manipulation and
technique of a painter may demand the peculiar quality
in a medium which oil in considerable proportion can
alone supply. A formula which answers well is this :
2 measures of copal oil-varnish made from Sierra Leone
or Zanzibar copal ;
1 measure of poppy oil ;
2 measures of oil of turpentine or oil of spike.
I40 MEDIUMS FOR OIL-PAINTING
By substituting linseed oil for the poppy oil a more
quickly-drying medium is obtained ; still more rapid
drying is secured by means of ' manganese ' oil. With
the same object in view, benzene may be used instead
of oil of turpentine. This latter ought, of course, in
all cases, to be one of the least resinifiable varieties
obtainable.
If an amber or copal varnish containing no oil be
available, a good medium may be prepared by mixing
3 measures of it with 2 measures of poppy or linseed oil,
and I measure of oil of turpentine or of spike.
Bell's medium contains no resin, but consists of thick-
ened linseed oil dissolved in oil of spike. To prepare it,
pure linseed oil may be oxidized by having a current of
warm, moist air passed through it until it has acquired
the consistence of fresh honey : this change occurs slowly
when a layer of oil is exposed to the air in a large flask,
the mouth of which is lightly plugged with carded cotton.
The flask should be shaken occasionally to mix its
contents, and to prevent the formation of a pellicle on
the surface of the oil.
Paintings executed with this thickened oil medium do
not acquire the hardness and solidity of those carried out
with a vehicle containing a hard resin.
Roberson's medium has now been in use for something
like seventy years. It has been and is a favourite vehicle
with many distinguished artists. A substitute for it is
sometimes prepared in the studio by mixing and warming
together strong copal varnish, poppy oil or linseed oil
and a trace of white wax.
Of megilp — a mixture of linseed oil and mastic varnish
— it is only necessary to say this : that however agreeable
as a medium with which to work, it contains a poor and
MEDIUMS FOR OIL-PAINTING 141
weak resin, which becomes in course of time yellow and
brittle, and is liable to be injuriously affected when a
picture, in which it has been used freely, is cleaned.
For painting in oil on plaster, slate, or stone, a perfectly
sound and convenient medium is made by warming 12
ounces of oil of spike, or of non-resinifiable oil of turpen-
tine in a glass flask plunged in water heated to the boiling-
point, and then pouring into it in a slender stream 4 ounces
by weight of paraffin-wax (melting-point about 58° to
62** C), or of ceresin, or of a mixture of these materials.
The mixture becomes perfectly clear if it be thoroughly
agitated and maintained at a temperature of 80° C. Then
20 measured ounces of ' picture '-copal varnish, or 16
ounces of oil-copal varnish, are slowly added, with con-
stant shaking, in the same way. It is of the highest
importance that the copal varnish used should contain a
sufficiency of oil. If a thin varnish with much terpene
be employed in preparing this medium the pigments may
lack coherence. I have found this defect counteracted
by using pigments ground in inspissated or blown linseed
oil, rather than in the medium itself. The * paraffin-
copal ' medium thus obtained may be diluted with oil of
turpentine exactly to the same extent as recommended
by the late Mr. Gambler- Parry, in the case of his ' spirit-
fresco ' medium, and may be used in the same way and for
the same purpose. Paintings executed with this medium
present a perfectly dead or matt surface without the least
shine. This medium is superior to that used in spirit-
fresco, for it contains neither elemi-resin nor wax, the
two doubtful constituents of the latter preparation, but it
is not so pleasant in use. Moreover, if a painting executed
with this medium on canvas be rolled up, the paint seems
to have some tendency to crack. This accident may be
142 SPIRIT-FRESCO MEDIUM
obviated if the rolling up and the unrolling afterwards
be carried out in a warm room.
Mr. Gambier-Parry's medium, to which reference has
just been made, is prepared with five ingredients. The
original instructions are unnecessarily complicated, and
may be simplified while keeping to the original propor-
tions, and without modifying the nature of the product
in the slightest degree. Eight ounces of oil of spike are
warmed in a glass flask to 80^ C, then 2 ounces by weight
of elemi are added, the mixture being warmed and shaken
till the elemi has dissolved. Some dirt and woody frag-
ments are sure to be introduced with the elemi, and so
the solution (still warm) must be filtered. Upon the filter,
when all the liquid has run through, 2 ounces by measure
of oil of turpentine, heated to 80° C, are now poured, and
the united filtrates are thoroughly mixed. The liquid is
then introduced into a flask, and heated to 80° C. ; then
4 ounces by weight of pure white wax (previously melted)
are poured in a thin stream into the solution of elemi and
thoroughly shaken. When the commixture is complete,
20 ounces by measure of ' picture '-copal varnish, or 16
ounces of oil-copal varnish, are gradually introduced with
constant agitation. The water surrounding the flask is
now made to boil, and kept boiling for five minutes. The
flask is withdrawn, wiped dry, and allowed to cool. As
the cooling proceeds the flask is gently- agitated from
time to time. When the mixture begins to get treacly in
consistence it is at once poured into the bottles (bottles
with wide mouths, holding 4 ounces apiece, are con-
venient) in which it is intended to preserve the medium
for use. The dilution of this medium and the mode of
using it are described in Chapter XXIII. on Painting
Methods.
PART III
PIGMENTS
Chapter XIII.— White Pigments Chapter XIV.— Yellow Pigments.
Chapter XV.— Red Pigments. Chapter XVI. - Green Pigments.
Chapter XVII.— Blue Pigments. Chapter XVIII.— Brown Pig-
ments. Chapter XIX. — Black Pigments. Chapter XX. — Classifi-
cation of Pigments. Chapter XXI. — Tables of Permanent and
Fugitive Pigments. Chapter XXII. — Selected and Restricted
Palettes.
CHAPTER XIII
WHITE PIGMENTS
Flake-White: White Lead — Ceruse — Blanc d* Argent —
Blanc de Plonih — Bleiweiss — Kremsevweiss.
White lead was known to the ancients. A face-powder
or cosmetic, found, in its original pottery-box of about
400 B.C., in the neighbourhood of Athens, proved to be a
mixture of white lead and whitening. Theophrastus,
Pliny, and Vitruvius describe its manufacture from lead
and vinegar. It was designated by several names, such as
cerusa, cerussa, cerosa, psimuthion. In the first half of
the fourteenth century it is mentioned as ' minium album.'
It has been called by divers names after the place or
method of its manufacture, or after persons who have
devised special processes for preparing it.
White lead still continues to be made for the most part
by processes which are essentially identical with the old
method, now generally known as the ' Dutch ' process.
This consists in attacking metallic lead, in the form of
'crates,' 'grids,' or spirals, simultaneously by acetic acid,
carbonic acid, atmospheric oxygen, and water-vapour.
The metal is gradually converted into a mixture or com-
pound of lead carbonate and lead hydrate. Other pro-
cesses, generally yielding an inferior product, containing
more carbonate and less hydrate, have been used. One
145 'o
146 FLAKE-WHITE
of these consists in passing a current of carbonic acid gas
through a solution of lead subacetate ; in another, 4 parts
of litharge, i part of common salt, and 16 parts of water,
are kept in contact for some hours with constant agita-
tion, and then carbonic acid gas is led into the mixture
until it becomes neutral to test-papers.
The best white lead contains two molecules of lead car-
bonate intimately associated with one molecule of lead hy-
drate, and is represented by the formula 2PbC03,PbH202-
This formula corresponds to about 70 per cent, of lead
carbonate, and 30 per cent, of lead hydrate. If the pro-
portion of hydrate rise above this percentage, the opacity
of the paint is lessened seriously ; if it fall much below
the above-named figure, the binding-power and working
quality of the white lead are impaired. Although the
normal lead hydrato-carbonate is probably constituted of
one molecule of each of its components, the formula
previously given may be taken to represent the highest
quality of white lead as a paint. It has been recently
stated that two molecules of lead hydrate associated with
three molecules of barium carbonate constitute a pigment
actually better than flake-white in some respects.
It is scarcely necessary to say that the metallic lead
used in the manufacture should be as nearly pure as
possible, such, for instance, as the lead from the Upper
Hartz, which contains but 2 parts of foreign metals per
1,000. These foreign metals, the presence of any one of
which in sensible quantity may cause a discoloration of
the product, are copper, bismuth, silver, cadmium, anti-
mony, nickel, and, more particularly, iron. But not only
must the raw material be pure, but it is necessary to
guard against the contamination of the white lead, during
its manufacture, by dust or sulphuretted gases.
WHITE LEAD
147
The impurities and defects of white lead are (i) acci-
dental, (2) intentional.
(i) Of the accidental impurities and defects of white
lead made from pure metal, the following are the chief:
a. Metallic lead, imparting a grey hue to the pro-
duct.
b. Massicot or litharge, the yellow oxide of lead.
c. Minium or red lead, which gives a rosy hue.
d. Excess of lead hydrate, which causes translucency.
e. Excess of lead carbonate.
/. Lead acetate.
A simple experiment will suffice to show whether lead
acetate be present in objectionable proportion in any
sample. Some of the dry pigment is to be ground with
distilled water into a paste, thrown on to a wetted filter
and then washed with freshly-boiled distilled water. The
clear filtered liquid should give nothing more than a slight
cloudiness on the addition of a little dilute sulphuric acid.
Some samples of flake-white which had been insufficiently
washed contained from 2 to 1 1 per cent, of lead acetate
removable by distilled water. In order to ascertain
whether the lead carbonate and lead hydrate exist in due
proportion in a sample of white lead, a weighed portion
of the dry pigment, after having been dried at 212° F.,
should be carefully roasted in a current of dry air, and
the water evolved (2 to 3 per cent.) intercepted by means
of a weighed calcium chloride absorption tube. This
operation, however, requires much manipulative experi-
ence, and, unless accurately performed, may lead to
erroneous conclusions.
(2) Of the intentional adulterations of white lead the
following are the most usual :
148 WHITE LEAD
a. Heavy spar, that is, native barium sulphate ; or
the same compound artificially prepared (per-
manent white, blanc fixe).
b. Gypsum.
c. China-clay.
d. Whitening or chalk.
e. Lead sulphate.
The first of these adulterations is by far the most usual.
Barium sulphate, in the form of finely -ground barytes, or
heavy spar, is the material employed on the large scale
for cheapening the cost of production of ordinary white
lead ; precipitated, that is, artificially prepared, barium
sulphate is used in the case of the finer makes of this pig-
ment. In either case the sophistication is very readily
recognised. Pure flake- white, for example, loses 14 J per
cent, of its weight when strongly heated so as to drive off
its carbonic acid and water, but * Venice ' white, which
is white lead and barium sulphate mixed in the proportion
of equal parts, loses, under such treatment, no more than
7*3 grains per 100. ' Hamburg' white, with 33 per cent,
only of white lead, loses 4*8 per cent., and ' Dutch ' white,
of which three-fourths are barium sulphate, gives off no
more than 3*8 per cent. * Crems,' or ' Cremnitz ' white,
is, or ought to be, pure white lead. The complete solu-
bility of pure white lead in dilute nitric acid may also be
made use of to detect the presence of barium sulphate,
which will remain undissolved as a dense white powder.
The adulterations with gypsum, china-clay, whitening and
lead sulphate, can be recognised only by further tests.
Gypsum, for instance, gives off water when heated, and
I part of it dissolves in 420 parts of water. China-clay
also gives off water when heated, but is insoluble in
water, and only slightly soluble in nitric acid. Whiten-
WHITE LEAD 149
ing dissolves in all mineral acids, but lead sulphate is
practically insoluble. After all, the detection of barium
sulphate is the only point with which the painter need
concern himself. It will therefore suffice if he ascertain
that a sample of white lead is of first-rate colour and
body, contains no sensible quantity of lead acetate, loses
when heated 14 J per cent, of its weight, and dissolves
perfectly in dilute nitric acid.
It has been observed that white lead is less liable to be
blackened by sulphuretted hydrogen and by other sul-
phides when it contains a small quantity of baryta-white,
or of lead sulphate thoroughly incorporated with it by
grinding. This observation opens the door to adultera-
tion, it is true : and it is perhaps wiser to rely upon the
protection furnished by resinous mediums and a final
coat of mastic varnish rather than upon any admixture
with other white substances.
The drawbacks attendant upon the use of white lead as
a paint are its poisonous character, its sickly and noxious
smell when used with oil, and its liability to discolour when
exposed to sulphuretted hydrogen or any sulphide soluble
in water. On the other hand the quality of the whiteness
of the best flake-white is unimpeachable: the paint works
admirably in oil, and has great body ; moreover, flake-
white not only mixes perfectly and safely with the majority
of permanent pigments, but it serves to impart to slow-
drying colours its own strongly siccative character.
Besides all these merits white lead possesses a valuable
property, which has scarcely been clearly recognised or
duly appreciated. For when an old oil-picture is carefully
examined, it will generally be found that if any portion of
its surface (of the paint, not the varnish) show decided
contractions and cracks, these are precisely those portions
150 . WHITE LEAD
into which white lead has entered in smallest proportion, if
at all. The most translucent parts, the rich glazings and
the deepest shadows may be fissured, but not the high
lights : examples illustrative of this point are referred to
in Chapter XXIV. of the present volume. This property
of white lead seems to depend upon a combination taking
place between a part of the oil with which it is ground and
a part of the lead hydrate which it contains. A degree of
toughness and elasticity is thus imparted to those films
of oil paint into which lead-white enters to any consider-
able extent.
Flake-white becomes brown, grey or black when ex-
posed to the action of sulphuretted hydrogen, ammonium
sulphide, or any metallic sulphide soluble in water. This
discoloration, which is due to the formation of lead sul-
phide, occurs more readily in the presence of moisture :
it is favoured by darkness to such an extent that a piece
of perforated cardboard laid upon a dry oil-painted
surface of white lead will, after a few weeks' exposure,
give a white pattern representing the perforations on a
buff ground, which corresponds to the solid parts of the
cardboard. But, after the removal of the perforated
card and subsequent exposure of the painted surface to
strong light, this pattern will disappear, the coloured
sulphide of lead being oxidized into the white sulphate.
The same change may be more speedily brought about
by means of a solution of hydrogen peroxide. By laying
a sheet of white filter-paper soaked in this liquid upon the
discoloured lead-priming of a prepared canvas the original
colour of the paint may be gradually brought back,
especially by the aid of a moderate degree of warmth.
This method is not available in the case of drawings or
water-colour paintings in which flake-white has black-
LEAD SULPHATE 151
ened ; but even these may often be successfully treated
by exposure to moist ozone, or by light touches of a
solution of hydrogen peroxide in ether. The latter treat-
ment has been successfully applied to a series of archi-
tectural drawings in gouache by C. Clerisseau in the Soane
Museum. In these the high lights had become black.
Old silver-point drawings, in which the lights were height-
ened with lead-white, may sometimes be thus restored to
their pristine state.
The specific gravity of the best flake- white is 6*6 ; 100
parts by weight of it require from 11 to 15 parts of linseed
oil in order to form an oil-paint of suitable consistence. It
is sometimes ground with poppy oil when a particularly
pure white product is demanded. The yellowish tint of
some makes of white lead is occasionally neutralized by the
addition of a trace of indigo or of artificial ultramarine.
Burnt or roasted white lead is sometimes used as a pig-
ment. It is of a cream-colour, a buff, or a pale yellowish
salmon, according to the temperature at which it has
been prepared, or the length of time during which it has
been heated.
Lead Sulphate. — Many attempts have been made to
utilize the sulphate of lead (PbSO^) as a pigment. This
compound, which is nearly insoluble in water and in dilute
acids, is almost, if not entirely, destitute of poisonous pro-
perties owing to this insolubility, although as ordinarily
prepared it possesses neither the pure whiteness nor the
body of white lead. But under the name of Freeman's
white lead, or non-poisonous white lead, a paint has been
introduced which may prove a rival to ordinary white
lead. It is essentially lead sulphate, and is prepared by
precipitating lead acetate solution with sulphuric acid.
But this precipitate is subjected to a special process of
152 ZINC- WHITE
grinding with small quantities of zinc-white and barium
sulphate, and acquires thereby a considerable increase of
density and opacity, although both the latter compounds
are of less specific gravity than the lead sulphate to which
they have been added. Being, when ground in oil, not
only destitute of the disagreeable smell of white lead, but
much less readily darkened by sulphuretted hydrogen.
Freeman's white possesses distinct advantages in these
respects over the more common paint. It may be mixed
with other permanent pigments without injuring them :
it is practically non-poisonous. On the other hand, it
does not possess the remarkable hardening and drying
powers of white lead.
Lead Oxychlovide. — Pattinson's white (PbClHO) does
not possess any advantage, as a white pigment for artists'
use, over the ordinary flake-white. Similar verdicts may
be pronounced as to the eligibility of several other white
compounds of lead, such as the antimonite, the anti-
moniate, and the tungstate of this metal.
The blanc d'argent of the French is supposed to be
pure lead carbonate free from any hydrate, but the great
majority of the specimens which I have examined are
nothing but flake-white of good quality. For general use
as a white pigment, both alone and in admixture, the best
flake-white, with all its defects, presents distinct advant-
ages over pure lead carbonate free from lead hydrate.
Zinc-white : Chinese white — Blanc de Zinc — Zinkweiss.
The substitution of carbonate of zinc for white lead
seems to have been first suggested by Courtois of Dijon
in 1787. After several unsuccessful attempts to introduce
either the carbonate or the oxide as an oil paint, the latter
began to be used about 1849-50, shortly after Leclairehad
ZINC-WHITE 153
shown how to prepare an oil suitable for making the paint
dry. We believe that it had been frequently employed as a
water-colour many years before 1849. So early as 1834
Messrs. Winsor and Newton prepared a peculiarly dense
form of this pigment under the name of Chinese white.
For the preparation of the best zinc-white it is essential
that the zinc be pure ; especially should it be as free as
possible from the metal cadmium. The zinc is heated to
the distilling-point in crucibles or retorts set in a furnace ;
the vapour, meeting with air, burns into the white oxide,
which condenses in a series of chambers. The contents
of these chambers vary somewhat in purity of tint ; the
presence of some metallic zinc generally imparts a greyish
hue to the zinc oxide nearest the crucibles or retorts. By
selecting the densest and whitest product, and then sub-
mitting this to powerful mechanical compression when
red-hot, an excellent pigment having a dense body is
obtained. Zinc-white prepared in the wet way, as by
the action of lime-water upon zinc chloride, is inferior in
substance to that made as above described, while that
obtained directly from blende is of bad colour.
As an oil-paint, zinc-white is a bad dryer. Instead of
being ground in raw poppy or linseed oil, an oil rendered
highly siccative by borate of manganese should be em-
ployed. In spite of its unquestionable merits, zinc-white
in oil cannot be recommended as a complete substitute for
flake-white. When used freely, it often shows a tendency
to crack and scale, besides becoming with age more trans-
lucent, or rather, less opaque. For water-colour painting,
tempera, and for fresco, zinc-white is practically perfect,
being unchangeable in hue or opacity under the most
adverse influences. Paper washed with zinc-white, either
alone or tinted with a coloured pigment, affords a good
154 BARYTA-WHITE
ground for silver-point, platinum-point, or pencil draw-
ings. There is a peculiar 'tooth' in the zinc-white which
freely brings off the metal or graphite from the pencil}
and serves to fix it on the prepared surface.
The purity of zinc-white is easily tested. Heated in a
tube, it should yield no volatile product, and should suffer
no permanent change of hue. It should dissolve com-
pletely without effervescence in boiHng dilute nitric or
hydrochloric acid. If, on heating, it acquires a permanent
yellowish hue, giving off moisture at the same time, white
lead is probably present. If it does not dissolve completely
in acid, it probably contains barium sulphate ; if efferves-
cence occurs during solution, either whitening, or white
lead, or zinc carbonate is present. Zinc carbonate, however
prepared, is inferior in whiteness and body to the oxide.
Zinc sulphide has been prepared as a paint ; its liability
to evolve sulphuretted hydrogen renders its use as an
artists' pigment dangerous, for there are several other
colours upon which it would exert a deleterious action.
It has very considerable body.
Baryta-white ; Permanent white — Blanc Fixe — Permanent
Weiss.
The mineral known as heavy spar, or barytes, has been
used as a white paint, particularly as an adulterant for
white lead. However finely it may be ground, it is always
very inferior in body and covering-power to the artificially-
prepared barium sulphate — the true blanc fixe. To make
this, a cold solution of barium chloride of specific gravity
I* 19 is prepared, and to it is gradually added in the cold,
and until no further precipitate is formed, dilute sulphuric
acid of 1*245 specific gravity. The barium sulphate is
MINOR WHITE PIGMENTS 155
washed with cold water until the wash-waters are entirely
free from acid ; for many purposes to which the product
is applicable (fresco and tempera painting) it should be
kept under water.
Baryta-white is absolutely unalterable by an impure at-
mosphere, and is without action upon other pigments. It
does not work well in oil, but a mixture of flake- white
and baryta- white, in the proportion of 2 to i, presents the
advantage of being very much less affected by sulphur-
etted hydrogen than flake-white.
The artificial baryta-white may be distinguished from
the natural by its much finer state of division, by its
greater body, and by the purity of its whiteness. Baryta-
white is not adulterated, but its almost absolute insolu-
bility in hydrochloric or nitric acid enables it to be at once
distinguished from zinc-white or white lead.
Several mixtures of barium sulphate and zinc sulphide
have been introduced as pigments ; they are not suitable
for the palette of the artist. The reaction by which the
majority of them are formed is brought about by mixing
together solutions of two soluble salts, barium sulphide
(BaS) and zinc sulphate (ZnSOJ, when two new salts
are precipitated, both insoluble, namely zinc sulphide
(ZnS) and barium sulphate (BaSO^).
Other white compounds used in painting are lime,
whitening, gypsum and China-clay. These have been con-
sidered in the chapter on Painting-Grounds. Amongst
white pigments which we need not describe are antimo-
nious oxide, antimonious oxychloride, lead sulphite, lead
tungstate, lead antimonite, and lead antimoniate. Not
only are these compounds difficult to prepare in a satis-
factory condition of purity and whiteness, but they are
liable to turn yellow or dull in impure air.
156 MINOR WHITE PIGMENTS
It should be stated here that the tests described in the
present chapter, and in all the other chapters on pigments,
refer only to the dry material, or, at any rate, to pigments
mingled with no fluid other than water. If it be desired
to operate on paints, this can be done, as a rule, only after
the removal of the vehicle with which they have been
ground. Oil may be removed by means of benzene or
turpentine-spirit, gum by treatment with distilled water.
CHAPTER XIV
YELLOW PIGMENTS
Yellow Ochre : Roman Ochre — Golden Ochre — Mineral
Yellow — Bro wn Ochre — Oxford Ochre — Ocre jaiine —
Gelber Ocker.
The distinction between the yellow ochres and the red
ochres, whether natural or artificial, depends upon a per-
fectly definite chemical difference. The colour of every
one of these pigments is due, indeed, to iron, and to iron
in the same state of oxidation ; but the iron oxide in the
yellow and brown ochres is chemically united to water,
while in the red ochres it is nearly or quite anhydrous —
that is, dry. In chemical language, then, we may say
yellow ochre is a ferric hydrate, red ochre a ferric oxide.
But, when we proceed to examine a number of samples
of yellow ochre, we find, not merely different proportions
of ferric oxide to combined water — that is, different ferric
hydrates — but we find also very variable proportions of
intruding or accessory constituents. In fact, yellow ochre
represents not less than three mineral species, and it
occurs associated with many impurities, the latter con-
sisting mainly of silica, of clay, of rocky debris, with
traces of gypsum, of iron or copper pyrites, and of humus
or peaty acids. There are, moreover, ochres in which
other compounds occur, as barium sulphate to the extent
157
158 YELLOW OCHRE
of 75 per cent, in some American varieties. The three
fundamental minerals, in order of frequency, which may
be traced in various yellow ochres, are these :
Brown haematite, or limomte, consisting of two mole-
cules of ferric oxide combined with three molecules of
water, and represented by the formula 2Fe203, 3H2O ;
Yellow haematite, or xanthosidente, consisting of one
molecule of ferric oxide combined with one molecule of
water, and represented by the formula FegOg, HgO ;
Bog-iron ore, or lymnite, consisting of one molecule of
ferric oxide and three molecules of water, and is repre-
sented by the formula Fefi^, sHgO; the separate exist-
ence and permanence of a hydrate having this fornmla
are, it must be owned, doubtful.
It is probable that all the numerous varieties of yellow
ochre, from the countless localities of this substance,
belong essentially to one or other of the above species of
iron minerals, although the frequent presence of such
impurities or accessories as silica, iron silicates, and clay
renders the identification very difficult. Moreover, there
are reasons for suspecting, in some ochres at least, the
presence of another and more complex compound, namely,
a distinct double iron-aluminium hydrate.
An analysis of a fine sample of yellow ochre, taken by
the author from a pit on Shotover Hill near Oxford, gave
the following percentages :
Hygroscopic moisture
- 7'i ;
Magnesia -
- 03
Combined water
- 9*0
Silica
■ 61-5
Ferric oxide
- 13-2
Calcium sulphate
- 14
Alumina - . -
- 6-3
Undetermined -
- 12
The varying hues of yellow ochres depend mainly upon
two differences of composition. One of these is the amount
of white clay, silica, calcium sulphate, or barium sulphate
YELLOW OCHRE 159
present in them — this lightens the colour ; the other is the
presence of ferric oxide, which gives them a ruddier or
warmer hue. All, when burnt— that is, calcined — lose
their essential water, and become converted into various
kinds of red ochre, light red, etc. The varieties which
contain much silica and clay (ingredients which, even in
good yellow ochres, often amount to two-thirds of their
weight) yield the less translucent and paler tints of some
of the burnt red ochres. India furnishes a great variety
of hues of yellow ochre, but our chief supplies come from
France, Italy, Germany and Spain. More recently excel-
lent ochres have been obtained from the district of Dubbo
in New South Wales. Some of the English ochres (from
Oxfordshire, Derbyshire, etc.) are of fine quality. Peri-
gord Yellow, a natural earth found in Perigord, is a fine
variety of yellow ochre : it yields when heated to 800°-
1,000° C. a fine reddish orange, brighter than that of the
light red produced from any other ochre.
Yellow ochre is generally prepared for use as a pigment
first of all by careful selection of the best pieces, and then
by the familiar process of elutriation, or washing over.
Thus it is at once freed from sand or other coarse particles,
and from any soluble salts which it may contain. Imme-
diately before being ground in oil, it should, however, be
dried at a temperature a little below that of boiling water,
as it is liable to contain hygroscopic moisture in addition
to its necessary constitutional water.
Yellow ochre is one of the most ancient pigments,
having been used by the Egyptians, the Greeks, and the
Romans. It is the oichra of Theophrastus. Pots of yellow
ochre were found at Pompeii. It has stood, with very little
change, the test of centuries. It certainly does become,
in all media, but especially in oil, slightly darker and
i6o YELLOW OCHRE
warmer in hue after prolonged exposure to light. The
change, however, is slight ; moreover, it soon comes to a
stop. It is probably due in part to a slight loss of con-
stitutional water from the ferric hydrate, and in part to
increased translucency. It must be recollected also that
yellow ochre as an oil-paint contains 40 or more per cent.
of oil, and this becomes yellower and darker in time.
Yellow ochre, so long as it is exposed to air and Hght, is
not darkened by sulphuretted hydrogen. It is without
action on other pigments, although the statement has often
been made, on quite insufficient grounds, that paints which
are damaged by contact with metallic iron are likewise
damaged by yellow ochre and by the red oxide of iron.
For instance, true Naples yellow is undoubtedly spoilt by
contact with a steel spatula, because the m.etal of the latter
takes away oxygen from, or ' reduces ' the lead antimoniate
of which the former consists. But such an action is
impossible with yellow ochre, for this iron compound is a
stable substance, containing already all the oxygen it can
take up. It is possible, notwithstanding, that ochre may
injure the hue of some lakes, such as yellow lake and
crimson lake, by replacing in part some of the alumina
with which the colouring matter is united. But as such
lakes are worthless, from their extreme instability when
exposed to light, when used alone, such probable action
of ochre upon them need scarcely be considered. Still
the same action may occur in the case of the madders
and alizarin pigments.
Yellow ochre is little subject to adulteration, for it is
too cheap a pigment to make it worth while to substitute
other substances for it. But sometimes the golden and
richer coloured varieties have been found to have had
their colour enhanced by the addition of certain fugitive
. YELLOW OCHRE i6i
or semi-permanent yellows of artificial or organic origin.
The majority of such additions may be detected by
pouring a little liquor ammoniae mixed with spirits of
wine upon some of the ochre placed on a filter-paper in a
funnel : the liquid passing through will be colourless if
the ochre be genuine. An ochre which when heated in a
test-tube gives off, besides water, fumes which partially
condense into a coloured or tarry matter on the glass,
contains organic matter, naturally present or artificially
added, and is generally of inferior permanence. Of late
years a far more frequent adulteration of yellow ochre is
the addition of chrome yellow — that is, lead chromate.
This adulteration may be detected by boiling the sus-
pected ochre with sodium carbonate solution, filtering,
and adding to the filtrate enough acetic acid to neutralize
it, and then a few drops of lead-acetate. A yellow pre-
cipitate indicates the presence of a chromate. An artificial
yellow ochre is made by acting upon solutions of iron
salts with metallic zinc, and thoroughly washing the
precipitate obtained.
Brown ochre is an approximately pure limonite : raw
sienna is very nearly related to it (see farther on), but
cologne earth, raw umber, Caledonian brown and vandyke
brown are distinct substances. An artificial brown ochre
is prepared by heating yellow ochre with 4 per cent, of
common salt to a low red heat.
Under the name of cyprusite a peculiarly bright lemon-
coloured earth has been imported from Cyprus as a
pigment : it consists essentially of a hydrated ferric
sulphate : it is not likely to prove a safe pigment for
artistic use.
i62 CADMIUM YELLOW
Cadmium Yellow: Orient Yellow — Aurora Yellow —
Daffodil — Orange Cadmium — Sulphide of Cadmium —
Javme Brillant—Jaune de Cadmium — Kadmiumgelb,
The metal cadmium, which is nearly related to zinc
both chemically and physically, was discovered by
Stromeyer in the year 1817. To one compound only of
cadmium, the sulphide, are due all the hues and tints
from the palest lemon cadmium to the fiery orange-red.
This compound is represented by the formula CdS, and
contains 112 parts by weight of cadmium to 32 parts of
sulphur. As commonly prepared, cadmium yellow is of
an orange hue ; when this compound separates slowly
from a solution, or is made in any way to take a dense or
3-ggregated form, it becomes of a decided reddish orange.
The orange-yellow variety, when very finely ground,
becomes less red and more incUned to yellow. Some of
the palest cadmium yellows contain white pigments, or
flour of sulphur, added to reduce their depth of colour :
the presence of free sulphur is sufficient to make any
pigment ineligible.
There are two well-known processes for making
cadmium yellow. In one of these pure cadmium oxide
is heated in a covered crucible with pure sulphur in
excess. In the other process, which yields pigments of
greater brilliancy and beauty, a soluble salt of cadmium,
such as the chloride or sulphate, is precipitated in the
presence of a little free acid, by means of a solution of
sodium sulphide, or preferably, of a stream of sulphuretted
hydrogen. The hue of the product inclines to red when
the solution is strong, hot and faintly acid ; to yellow
when it is weak, cold, and neutral. It is necessary to
state that all the materials used must be pure. Iron,
CADMIUM YELLOW 163
lead, bismuth, and any metals giving a coloured sulphide,
even in traces, are seriously detrimental to the beauty of
the product. The precipitate of cadmium sulphide, after
having been thoroughly washed with boiling distilled
water until the wash-waters no longer redden blue litmus
paper, is collected on filter-papers and dried in the water-
oven. In order to remove any free sulphur that may be
present, the dry cadmium yellow may now be digested in
a suitable vessel with pure carbon disulphide. After this
treatment the pigment is once more dried, and is then
ready for grinding in oil or other vehicle. Cadmium
yellow, prepared by the process last described, presents
a satisfactory degree of permanence, and has no action
on white lead when both pigments are ground together
in oil. But a curious change has been noticed when the
orange-red variety of this pigment, ground in oil, was
kept some time in the ordinary metallic collapsible tubes,
which formerly contained some lead, although of late
years they have been made of nearly pure tin. The
interior surface of the tube became darkened, sometimes
almost black, from the formation of lead sulphide. It is
certainly strange that a similar action does not occur
between white lead and these deep cadmiums. For I
found that the same sample of cadmium-red in oil which
had blackened the metallic tube, when some of it was
laid upon flake-white in oil, and kept for years, had not
darkened the lead compound anywhere, even at the
surface of contact. Moreover, cadmium yellows mixed
with flake-white prevent, as do many other substances,
such as baryta-white, lead sulphate, etc., the ready
darkening of this lead paint by sulphuretted hydrogen.
On the other hand, the cadmium yellows act with great
energy upon some of the pigments containing heavy
i64 CADMIUM YELLOW
metals. Emerald green, for example, is rapidly ruined
by cadmium sulphide, both in water and in oil ; cadmium
yellow and emerald green (Schweinfurt green) are abso-
lutely incompatible. Chrome yellow and true Naples
yellow are also darkened by admixture with cadmium
yellow, at least after a time. With oil colours, a sample
of yellow ochre, which was afterwards found to have
been adulterated with chrome yellow to the extent of
8 per cent, became yellowish-grey after admixture with
some cadmium yellow.
While the stability of what may be called the normal
cadmium yellow or orange is pretty well assured, both as
an oil and a water colour, a very different verdict must be
pronounced upon pale and lemon cadmium when used in
water-colour painting. When thus used these pigments
do not merely fade, but acquire a somewhat greyish hue.
The following observations throw some light upon these
changes. During the year 1876 I prepared a number of
samples of cadmium yellow and orange. All were obtained
by the action of sulphuretted hydrogen upon solutions of
cadmium chloride. The products ranged in hue from a
lemon colour to a deep orange, according to the strength
of the solution, the presence or absence of free acid, and
the temperature at which the precipitation of the pigment
took place. After due washing and drying the various
samples were put into bottles and preserved in my labora-
tory. They were never exposed to direct sunshine. On
examining them from time to time it was noticed that the
specimens of medium depth, having a yellowish orange
hue, kept their hue perfectly, while two or three of the
orange-red varieties exhibited a curious phenomenon of
alteration. The loose friable lumps into which the powder
had aggregated were distinctly paler on the outside than
CADMIUM YELLOW 165
in the interior, while the parts of the contents of the
bottles which had been most exposed to light were paler
than those which had been comparatively shaded. But a
still more marked change had taken place in the samples
to which the term ' pale ' cadmium might be applied.
These had generally become still paler, almost straw-
coloured, especially where most exposed to light ; but in
some of the specimens orange specks were observed,
resembling in hue what is usually called ' middle ' cad-
mium. From the above observations it would seem that
there is a tendency in differently tinted * wet process '
cadmium yellows to return to what we may call the
normal or medium hue, but that the palest varieties are
most subject to change. This change seems to arise in
part from oxidation and hydration, for the bleached speci-
mens gave indications of containing some white cadmium
hydrate, when heated giving off a little water, and be-
coming brownish from the formation of the brown oxide
of cadmium. Such a bleaching of pale cadmium, if my
explanation be correct, is in a measure explicable if we
recollect that this variety occurs in a very fine state of
division, and on this account is more liable to chemical
change. In water-colour painting, where there is no
effective protection through the presence of a hydrofuge
medium, this fading of ' wet process ' pale cadmium is
notorious. In oils this cadmium, like the others, is generally
thought to be permanent. My faith in the inalterability
of cadmium pigments, even in oil and allied media, has,
however, been somewhat shaken during recent years.
Cadmium orange has almost perished where used in
Leighton's lunette * Arts of Peace,' in the Victoria and
Albert Museum, a work executed in spirit-fresco. I
regard the passage of the pale and of the deep cadmium
1 66 CADMIUM YELLOW
yellow when in powder into the normal or middle variety
as dependent chiefly, if not entirely, upon molecular
changes. Moreover, the pale cadmiums are rarely found
free from admixture, and their alterability may be in part
owing to the foreign ingredients they contain. More
recent researches by G. Buchner and N. von Klobukoff
confirm the conclusions drawn from my early experiments.
There can be no doubt that cadmium sulphide exists in
two if not in three molecular states, differing not only in
colour but in crystalline form and in specific gravity.
Thus pale cadmium has the specific gravity 3*9 to 4*5,
while the red modification is denser — 4*5 to 4-8. And
when the pale variety, dry and in powder, is rubbed
strongly with a piece of agate, its colour deepens and
reddens in a very decisive manner. The same change
occurs when a water-colour wash of cadmium yellow is
exposed for a year or so to sunlight in a perfectly dry
atmosphere. This phenomenon is clearly analogous with
that shown when the yellow mercuric iodide is altered
into the scarlet form by pressure. It is perhaps safer to
employ an ivory palette knife rather than one of steel in
manipulating the cadmium pigments.
Aurora yellow is a bright and beautiful pigment con-
sisting essentially of cadmium sulphide. It has more
opacity than most of the other varieties of cadmium and
possesses a pure yellow hue. Its stability is greater than
that of many other varieties of this pigment. Daffodil
yellow is the name given to another variety of cadmium
sulphide, prepared at a red heat and containing a small
quantity of magnesia. Neutral orange is a mixture of
cadmium yellow with Venetian red.
Cadmium yellows are sometimes adulterated with
Indian yellow, baryta and strontia chromates, and
CADMIUM YELLOW 167
chromates of lead. Indian yellow shows its presence by
blackening and giving off tarry fumes when the pigment,
in the state of dry powder, is strongly heated in a test-
tube. The chromates may be detected by the green colour
produced when the sample is warmed with alcohol and
dilute sulphuric acid. The lead chromates or chrome
yellows, and the orange and red basic chromates of the
same metal will blacken when the substance in which
they are present is moistened with weak ammonium
sulphide. Free sulphur in pale cadmium yellows comes
off as a vapour when the sample is heated, but it may be
better detected by the solvent action upon it of carbon
bisulphide. Baryta-white may be detected by its insolu-
bility in hot strong hydrochloric acid, in which cadmium
sulphide dissolves.
Cadmium red and cadmium orange are slightly trans-
lucent when compared with the paler and yellower varie-
ties of this pigment, and possess very full and glowing
hues. They work well as oil and water colours. Mixed
with zinc-white or flake-white, deep and middle cad-
miums yield several beautiful colours, some of which
closely resemble the different varieties of true Naples
yellow, and are now employed very largely in lieu of the
latter pigment. Pure cadmium yellow, when heated
moderately, becomes orange-red or red, but regains its
pristine hue on cooling. If, however, the heat be con-
siderably raised in the presence of air, some of the sulphur
in the compound burns, and the residual mass presents
a dull brown colour. ' Manganese oil ' accelerates the
drying of the cadmium colours, which is sometimes
inconveniently slow.
168 A UREOLIN
AuREOLiN : Cobalt Yellow — Jaime de Cobalt — Kobaltgelb.
Origin and Composition. — This remarkable artificial
yellow pigment was discovered by Fischer. It is a com-
pound of the nitrites of cobalt and potassium. Usually
it is free from water, but it sometimes contains three
molecules, and is then represented by the formula
KgCo2(N 02)12, 3H2O. Other proportions of water also
occur ; but when the compound contains four molecules,
its hue is somewhat greenish. The dry or anhydrous
variety is best made by mixing a solution of a cobaltous
salt, strongly acidified with acetic acid, with a concen-
trated solution of potassium nitrite, and keeping the
mixture warm. Perhaps a pigment of finer hue is
obtained by passing a stream of nitric oxide gas mixed
with air into a solution containing nitrate of cobalt and
a little acetate of potassium ; from time to time a little
potassium carbonate is added.
Another method of preparing a variety of aureolin
having a singularly bright yellow hue consists in adding
a solution of sodium cobaltinitrite acidified with acetic
acid to a dilute solution of potassium acetate or nitrate.
I have tried the experiment in accordance with the in-
structions given by Messrs. Adie and Wood (in the
' Transactions of the Chemical Society,' vol. Ixxvii., 1900,
p. 1076), but I have used sometimes other cobalt salts
instead of the acetate, and have also so arranged the
constituents of the two solutions that the precipitate of
aureolin is intimately associated w4th barium sulphate
precipitated at the same time. By this means an opaque
pigment of bright and light yellow hue is obtained. The
pure aureolin obtained by this process contains sodium
as well as potassium, and is represented by the formula
A UREOLIN 169
K2NaCo(N02)6,H20. Its value as a pigment is at least
equal to that of the better-known varieties of aureolin.
The pure pigment prepared in the way indicated, without
any suggestion of its use in painting, by Messrs. Adie
and Wood, requires less oil than usual and dries well ; in
water-colour painting it shows one distinct advantage
over the older varieties of aureolin, for it is less soluble
in water and does not sink into the paper. The several
varieties of aureolin are not much affected by caustic
potash solution or by dilute hydrochloric or nitric acid,
and are very slowly attacked and blackened by solution
of sulphuretted hydrogen, but are at once destroyed by
ammonium sulphide. Ordinary aureolin is slightly soluble
in cold water.
Aureolin is of a pure yellow colour, and is almost trans-
parent whether used in water or oil painting. In oil some
samples dry with great difficulty, and become very dirty
if exposed to the air during the progress of desiccation ;
other samples dry and harden too quickly — the exact
cause of this difference of deportment has not been
ascertained. Moreover, as ordinarily ground in oil, some
varieties require a very large proportion of the medium.
These defects may be easily remedied by heating the
slow-drying variety of the ground pigment to 212° F.
immediately before the addition of the oil, and by using,
instead of raw linseed oil, the siccative linseed oil, pre-
pared by means of borate of manganese. Thus prepared,
aureolin not only dries quickly, but it retains its purity
of hue ; moreover, a surface of the dried oil pigment will
yield nothing to a wet cloth passed over it instead of
staining it yellow. The reason why this staining occurs
with ordinary aureolin ground in oil, even when it has at
last become dry, is that the oil does not suffice to pro-
I70 AUREOLIN
tect the particles of pigment from the solvent action of
moisture. The quick-drying variety of aureolin should
be ground in poppy oil.
I find that the variety of aureolin which contains sodium
as well as potassium (see above) dries perfectly well when
ground in purified linseed oil, even when the latter has
not been made siccative by special treatment.
Aureolin properly prepared in oil, as described above,
does not fade by exposure to sunlight, nor does it darken,
except so far as the admixed oil is concerned. As a
water-colour aureolin is practically permanent, even in
sunlight, as the following figures show :
Original intensity ... ... ... lo
After two and five years ... ... lo
After ten years ... ... ... 9
In the Burlington Club trials it was found that aureolin
stood perfectly for four years when exposed in an ordinary
frame or in air kept dry, but that it lost somewhat by
exposure, during the same time, in a hermetically sealed
tube in the presence of ordinary moist air.
The fading of fugacious organic pigments, such as the
lakes from cochineal, is accelerated by their commixture
with aureolin, which particularly hastens the destruction
of indigo, even in oil. The aureolin cannot so act without
being itself likewise affected ; it generally becomes, under
such circumstances, of a brownish hue.
It is easy to learn whether a sample of aureolin is free
from combined water by heating a small portion some-
what strongly in a long test-tube; dew will condense
upon the upper part of the tube if water be present in the
pigment. The presence of yellow organic matters in
imitative or adulterated aureolins may generally be
LEMON YELLOW 171
detected by mixing some powder of the suspected sample
with spirits of wine and a few drops of strong ammonia ;
the liquid becomes red orange, or yellow if the aureolin
be not pure. Aureolin containing chrome yellow is
blackened by a solution of sulphuretted hydrogen.
Aureolin is the first pigment described (in the present
manual) which illustrates the remarkable colouring power
of the element cobalt. The hues derived from this metal
acting as a chromogen range from yellow to green, blue
and violet or purple ; there is also a rose cobalt. Various
oxides, themselves colourless, serve as chromophores.
Lemon Yellow : Baryta Yelloiv — Barium Chromate —
Yellow UUraniavine — Permanent Yellow — Jaime d'Outve
mer — Zitronengelb.
Of all the chromates which have been used in painting,
barium chromate is the most stable. It has a pure
yellow colour, with a not inconsiderable degree of opacity.
It works smoothly.
Lemon yellow is often made by mixing solutions of
neutral potassium chromate and of barium chloride, both
liquids having been previously heated to 100° C. A still
better plan is to take equivalent proportions — namely,
25I parts by weight of pure crystals of barium chloride
and 21 J parts of pure crystals of neutral potassium
chromate — of these two compounds, and to grind them
together to very fine powder. Continue the grinding, and
then add gradually sufficient pure water to convert the
mixture into a thin paste. The paste is then heated to
100° for fifteen minutes, thrown on a filter, washed with
abundance of pure water, dried, and ground.
Properly prepared lemon yellow may be mixed with
most other stable pigments without suffering change. It is
172 GAMBOGE
not blackened like the lead chromes by sulphuretted
hydrogen, but it has a tendency, as a water-colour, to
become greenish when long exposed to this gas or to
impure air. In oils it is very useful, for although some
organic pigments may give it a greenish cast by reducing
it in part to green chromic oxide, yet it may be safely
associated with aureolin, with madder carmine, and with
Prussian blue. Lemon yellow may be used in fresco.
Strontium chromate is very often — we may say gener-
ally — substituted for true lemon yellow, but it is less
stable, and has the further defect (for water-colour work)
of being decidedly soluble even in cold water, so that
light washes of it may be found to sink into the paper
and to partially disappear. The most common adultera-
tion of lemon yellow is with pale chrome ; of course,
sulphuretted hydrogen detects this falsification by darken-
ing or blackening the pigment. Strontium chromate is
distinguished from barium chromate by its dissolving in
boiling water to such an extent as to yield a solution
having a strong yellow colour. It may be prepared in
the same way as the chromate of barium. Zinc chromate
and calcium chromate are yellow pigments of inferior
value. A mixture of zinc chromate with barium chromate
is sold as primrose yellow.
Gamboge : Gomme-gutte — Gummigutt.
Origin. — This gum-resin is produced by several species
of Garcinia. Siam gamboge comes from G. Hanhuryi
(Hook, f.) ; Ceylon gamboge from G. Morella (Desv.).
There are other species from which the same product is
obtained in various parts of India, as G. Camhogia
(Desrouss.) and G. elliptica. The fine, deep-coloured
GAMBOGE 173
gamboge, produced by the Burmese G . heterandra (Wall.),
may prove to be superior to Siam gamboge, but it has
not yet become an article of European trade. Gamboge
is a mixture of a gum soluble in water, and a resin which
is soluble in alcohol, chloroform, ether, etc. The pipe-
gamboge of Siam, which is as pure as any variety met
with in commerce, contains about 78 per cent, of resin
and 18 of gum. The resin, which is the true colouring-
matter, may be easily obtained pure by crushing pipe-
gamboge into fine powder, mixing it with a little water,
and then shaking up the mixture with ether ; the ether
dissolves the resin alone. From the ethereal solution the
colouring-resin is recoverable by evaporation ; but it is
better to add a little drying-oil and some copal- varnish
before driving off the ether by means of a very gentle
heat. The coloured, semi-fluid mass which then remains
may be preserved in bottles or tubes for use as an oil-
paint. The resin of gamboge has the properties of an
acid, and forms yellow, orange, or brown compounds,
with soda, lime, baryta, and other bases. Some of these
compounds might prove useful as paints.
Gamboge was used by the early Flemish oil-painters.
In the seventeenth century it was largely employed to
give a golden hue to the embossed leathers for which
Amsterdam was famous.
In water-colour painting gamboge is not trustworthy.
It is unaffected by sulphur compounds, but is darkened
by ammoniacal fumes, and slowly bleached by strong
light. Some samples prove, however, far less fugitive
than others. In two years' exposure to sunlight, one
sample of cake-gamboge lost more than half its original
intensity ; while a sample of moist gamboge, bought at
the same time from the same maker, retained nine-tenths.
174 INDIAN YELLOW
The same sample of moist gamboge, after seven years,
still showed seven degrees out of the original ten of
intensity.
As an oil-colour, gamboge affords a rich, transparent,
golden or amber hue ; it has some claims to the con-
sideration of artists. To secure its permanence, ad-
mixture with oil alone does not, however, suffice ; a resin
such as copal, or Strasburg turpentine, or wax, or
paraffin, must be used also. Some of Sir Joshua
Reynolds' trials of gamboge prove this, those with oil
alone being a name only now ; while those with resin, or
wax, retain their original hue very fairly, though they
were spread upon the canvas in 1772. It must, there-
fore, be remembered that reliance cannot be placed upon
the permanence of the ordinary gamboge oil-paint as
met with in commerce.
Gamboge, from its resinous nature, shows, when laid
on thickly as a water-colour, a rather shining surface.
It appears to have little or no chemical action on other
pigments (with the exception, perhaps, of white lead),
although, if it be mixed with anything which contains
lime, or other alkaline compounds, it becomes brownish,
and darkens. Gamboge forms beautifully clear and rich
greens with Prussian blue or indigo, but its place in
water-colour painting may be advantageously taken by
aureolin, and even by Indian yellow. When mixed with
baryta yellow or cadmium yellow, the permanency of
gamboge is enhanced.
Indian Yellow : Piuri, Ptirree, Peori — Jatme Indien —
Indischgelb.
This remarkable pigment is obtained at Monghyr, a
town in Bengal, from the urine of cows which have been
INDIAN YELLOW i75
fed upon mango-leaves. It generally occurs in the
bazaars of the Panjab in the form of large balls, having
an offensive urinous odour.
Indian yellow is an impure magnesium salt of euxanthic
acid. The essential part of it is a compound containing
4*5 per cent, magnesia, 187 per cent, water, and 787
per cent, euxanthic anhydride; but this substance is
always associated, even in the most carefully purified
samples of prepared Indian yellow, with various im-
purities both mineral and organic. The pure mag-
nesium euxanthate is represented by the formula
CisHieMgOii, 5H2O.
For artistic purposes the crude imported Indian yellow
is thoroughly powdered, and then washed with boiling
water, until the liquid filtered from it is no longer
coloured ; a brown impurity, and much of the evil smell,
are thus removed. The colour of the washed product is
enriched by leaving it in contact for a day or two with a
saturated solution of sal-ammoniac, and then repeating
the treatment with hot water.
Thus purified, this pigment presents a translucent
orange-yellow colour of great depth and beauty. Ground
in oil, some specimens are practically unchanged, even
after long exposure to sunlight, any darkening they show
being due either to imperfect purification, or to the
change of the associated oil. Such change is reduced to
a minimum if poppy oil be substituted for linseed oil, or
if the latter be previously treated with manganese borate.
On the other hand, I have met with specimens of Indian
yellow ground in oil which, after five years' exposure,
have lost nearly one-third of their original depth, and
have, at the same time, become rather reddish-brown in
hue. As a water-colour, Indian yellow retains its hue
176 INDIAN YELLOW
unimpaired when exposed to diffused daylight ; sunlight
very slowly bleaches it, the hue it acquires being some-
what brownish. The rate of alteration and of reduction
in force caused by sunlight may be approximately repre-
sented by these figures :
Original intensity lo
After 2 years
After 5 years
After 7 years
After 10 years
When this water-colour pigment is exposed to sunlight
in the presence of air maintained in a state of perfect
dryness it loses its colour much more rapidly than under
ordinary conditions. For this reason it may be advisable
to incorporate an extra proportion of glycerin with Indian
yellow when prepared as a water-colour.
As a general rule, Indian yellow suffers no change by
admixture with any pigment itself permanent, nor is it
affected by sulphur compounds. True Naples yellow,
however, most of the chromates, and probably aureolin
alsO;, tend to embrown it to some extent.
Indian yellow which has been adulterated with lead
chromate (chrome yellow) becomes dark-brown when
moistened with ammonium sulphide.
A fine yellow pigment may be prepared from the
euxanthic acid, which is the characteristic constituent of
Indian yellow, by throwing it down in combination with
the two bases — alumina and magnesia. The following
directions may be followed : Dissolve i part of pure
euxanthic acid in just sufficient dilute ammonia. Pour
the solution into a liquid prepared by dissolving 45 parts
of potash-alum, 15 parts Epsom salts, and 6 parts sal-
MARS YELLOW 177
ammoniac in 250 parts of water. Now cautiously add
dilute ammonia to the mixture, stirring all the time, and
avoiding any excess of ammonia. The precipitated pig-
ment is to be thoroughly washed, and then pressed, dried,
and ground.
Mars Yellow: Mars Orange — Artificial Ochre — Jaime
de Mars.
This pigment is a kind of yellow ochre prepared arti-
ficially. It may be made by precipitating a salt of iron
mixed with alum by means of caustic soda, or potash,
or lime. The salts of iron used are either green vitriol
(ferrous sulphate) or the ferric chloride. If green vitriol
be employed the precipitate formed gradually becomes
yellow on exposure to the air. Upon the proportion of
alum mixed with the iron salt depends the depth of the
yellow colour in the product, for the alumina precipitated
with the iron hydrate acts as a diluent of the colour.
When lime is used as a precipitant for the iron com-
pound (if this be green vitriol or ferric sulphate), calcium
sulphate, that is, gypsum, comes down along with the
ferric hydrate and basic ferric sulphate, and serves to
lighten the colour.
By submitting the different varieties of Mars yellow
to various degrees of heat, with or without a little nitre,
a number of products of different hues are obtained, in-
cluding Mars orange. Mars red, Mars brown, and Mars
violet. All these preparations require very thorough
washing to fit them for use on the palette of the artist.
The Mars colours are permanent when carefully pre-
pared and thoroughly purified from soluble salts. They
seem sometimes to have a slightly injurious effect upon
a few of the best semi-permanent pigments of organic
12
178 NAPLES YELLOW
origin, such as the madder colours. This action may be
due to the ferric hydrate in them combining with the
colouring matter, and displacing some of the alumina
previously united with it. In this direction it is probable
that Mars yellow will be more active than the deeper-
coloured pigments produced by calcining it at various
temperatures.
Naples Yellow : Jaune de Naples — Jaune d'Antimoine
— Neapelgelh — Giallo di Napoli.
Under this name three different substances are in-
cluded. The pigment generally sold in England as
' Naples yellow ' is an excellent imitation made by mixing
cadmium yellow or deep cadmium with a white, prefer-
ably a zinc white. But a true Naples yellow, which is
a basic lead antimoniate, is still procurable from some
artists' colourmen. This preparation is sometimes made
by heating together for two hours a mixture of i part
tartar emetic, 2 parts nitrate of lead, and 5 parts common
salt, all the ingredients being of the purest quality, and
the heat not exceeding that at which common salt fuses.
A more recent process, in which zinc oxide is introduced
among the materials which are heated together, yields a
paler but excellent product. K bright pale variety of
yellow ochre seems to have formerly gone under the
name of Naples yellow.
This antimonial yellow has been known from very early
times as an enamel colour. It has been found upon
Babylonian bricks at least 2,500 years old. Persian
pottery as early as the thirteenth century of our era is
occasionally decorated with antimonial yellow.
In oil the genuine and the imitative Naples yellows
NAPLES YELLOW 179
are quite permanent, so far as light is concerned, but the
genuine kind is liable to be darkened, like other lead
compounds, by air containing sulphuretted hydrogen. In
water-colour painting genuine Naples yellow is quite in-
admissible, for it blackens rapidly, but irregularly, in the
presence of mere traces of sulphur compounds. This
blackening, like that of lead white under similar con-
ditions, is much more marked in darkness than in light.
Naples yellow, in contact with metallic iron, tin,
pewter, zinc, and several other metals, is discoloured and
blackened. An ivory instead of a steel spatula, or palette
knife, should be used with this pigment. The darkening
in question is due in part to attrition, owing to the
extreme hardness of the particles of the lead antimoniate,
however finely the material may have been ground, and
partly to the reducing effect of the above-named metals
upon this antimoniate. Iron in the form of its oxide or
hydrate (as in light red or yellow ochre), or in complex
combinations (such as Prussian blue), does not exert any
effect upon Naples yellow. A statement to the contrary
effect has crept into a large number of technical manuals,
but I have been unable to discover the slightest experi-
mental evidence in favour of such a view. Naples
yellow, however, is injured by and does injure some of
the organic pigments, such as the cochineal reds and the
numerous yellow lakes. But as Naples yellow cannot
be used as a water colour, and as the above-named
organic pigments ought to be entirely excluded from the
palettes of all artists, the action in question is of little
importance. Naples yellow acts upon indigo also.
Indigo, however, is a pigment, to which a very high
degree of permanence cannot be assigned ; there is,
moreover, no reason why it should be associated with
i8o YELLOW LAKE
Naples yellow, as other yellow pigments may be safely
used to modify its hue.
Another pigment also is sold as jaune d'antimoine. It
is a mixture of the oxychlorides of bismuth and lead with
lead antimoniate. When carefully prepared it yields a
rich paint of good body, but its use cannot be recom-
mended to artists.
Yellow Lake : Brown Pink — Citrine Lake — Yellow
Madder — Italian Pink — Quercitron Lake — Gelhev Lack.
Origin. — The sources of yellow lake are numerous, but
the best kind is obtained from quercitron bark from
Quercus tinctoria, Qu. nigra, and Qu. citrina, three species
of North American oak. A hot-water decoction is made,
and this is precipitated by a solution of alum and dilute
ammonia. A richer yellow pigment is obtained by
extracting the powdered bark and alburnum with boiling
dilute sulphuric acid instead of with water. The original
colouring matter of the bark (quercitrin) is thus changed
into a more stable compound known as quercetin. The
former substance is a glucoside, the latter has the
character of an acid ; both may be converted into lakes
by bringing them into contact with precipitating or pre-
cipitated hydrate of alumina. Yellow lake was formerly
made from the fruits of various species of buckthorn,
known as Persian, Turkish, or Avignon berries. The
species yielding these fruits are Rhamniis infectorius,
R. oleoides, R. saxatilis, R. amyi^dalinus, R. catharticus. The
bark of R. frangnla and of R. catharticus also yields a
yellov/ pigment. * Stil de grain,' and several of the con-
tinental yellow lakes, are made from the above-named
berry.
BROWN PINK
i8i
Italian pink, Dutch pink, and deep yellow madder are
names usually given to the richer yellow lakes of quer-
citron, although some of these pigments are occasionally
prepared from Turkish or Avignon berries.
Beautiful and useful as many yellow lakes undoubtedly
are, they should be rigorously excluded from the artist's
palette. In oil most of them are very bad driers, as well
as fugitive : in water-colour they generally lose nine-
tenths of their colour within two years of exposure to
sunlight : the residual stain is ultimately of a bluish-grey.
The following observations as to the behaviour of
several members of this group, on exposure for two years
to sunlight, apply to the colours as ground in oil, and as
mixed with flake- white in tint :
Name
/« Oil only
With
Flake- white
Original
Intensity = 10
Laque Robert — hell-
gelb - - -
Lemon yellow •■
Pale straw -
- 2
^
Laque Robert— dun-
1
kelgelb -
Deep lemon
Stone -
- 4
8
Laque brun-jaune
Salmon -
Pale rose -
- 7
*«
Laque brun-fonce
Yellowish -grey
Smoke-grey
- 8
1
Pale yellow madder -
Pale orange
Pale buff -
- 7
K
^
Deep yellow madder -
Greyish salmon
Pale greyish
pink 6;
The same pigments used as glazing colours over flake-
white have faded to about the same extent, but their
change of hue is, in one or two cases, rather less marked.
The so-called brown pink is usually a deep quercitron
lake, although it was formerly made from the berries of
one of the kinds of buckthorn (Rhamnus) previously
named. I have never met with a specimen of it which
would stand a year's exposure to sunlight without suffer-
ing almost complete change or loss of colour both in Vv^ater
and in oil. And it further presents the awkward effect of
1 82 CHROME YELLOW
becoming ultimately of a cool bluish-grey hue, a change
particularly unfortunate when it has been freely used to
represent foreground vegetation, or the golden lights on
the near foliage of trees. Yet I am bound to confess
that in Mr. W. Simpson's fifteen years' trial of certain
water-colour pigments, the brown-pink has suffered com-
paratively little alteration. Had a portion of the original
cake-colour employed been preserved for examination it
might have been possible to have discovered the cause
of this anomalous behaviour of the particular specimen
in question.
Chrome Yellow : Chrome — Chromate of Lead — Jaune de
Chrome — Chromgelb.
This pigment, when of a pure yellow hue, is the neutral
lead chromate. By associating it with an additional
quantity of lead oxide it may be obtained of various
orange and reddish orange hues. It may be made by the
mutual action of a soluble lead salt, such as the acetate
or nitrate, and the chromate or bichromate of potassium.
Or white lead in fine powder (2 kilos.) may be boiled with
a solution of bichromate of potassium (J kilo.) in water
(10 litres). Alum and baryta-white, or lead sulphate, are
also employed in the preparation of some of the paler
chrome yellows. Lemon chrome is a mixture of lead
chromate and sulphate. Orange chrome and chrome red
are prepared from a mixture of lead acetate (6f kilos.),
litharge (5J kilos.), neutral potassium chromate (6 kilos.),
caustic potash being sometimes used in addition. Chrome
red may be obtained also by the direct action of caustic
soda in solution upon the yellow lead chromate : its
chemical formula is PbCrO^, PbO, or Pb2Cr05.
VANADIUM YELLOW 183
The chromates of lead are peculiarly liable to change,
and are quite unfitted for use in tempera or water-colour
painting. In oil, especially if protected by varnish, or
locked up in a resinous vehicle, these pigments show a
certain measure of permanence, except when they are
mingled with paints of organic origin. In fact there are
two causes which militate against the integrity of the lead
chromates. One of these is the tendency which they
possess towards reduction, that is, the loss of oxygen by
their chromic constituent, by which the green or lower
oxide of chromium is formed. This change is brought
about by many kinds of organic matter, notably by such
animal or vegetable pigments as are themselves prone to
oxidation. The other cause of deterioration is the pres-
ence of certain sulphur compounds which act upon the
lead chromates in the same way as they act upon white
lead, producing lead sulphide of a dark brown, or a grey
colour.
Of late years the respective merits as oil-paints of
cadmium yellow and chrome yellow have been warmly
contested ; the tendency at present, especially among
artists rather than among chemists, is to give a verdict
in favour of the latter pigment.
Vanadium Yellow.
It has been proposed to employ the beautiful golden-
bronze crystals of meta-vanadic acid as a pigment. They
possess, when finely ground, an intense colour, like that
of a very rich golden ochre, but less earthy, and more
brilliant. This pigment has remarkable covering power,
and works admirably both as an oil and a water-colour.
Although the material is somewhat costly, the price for
which it could be prepared need not preclude its use.
1 84 KINGS' YELLOW
But, unfortunately, this colour is not permanent. A few
hours' exposure to sunshine of a water-colour wash of
vanadium yellow suffices to change and deteriorate its
hue in a marked degree.
Kings' Yellow : Orpinient — Jaime Royal — Konigsgelb,
The yellow arsenious sulphide (AsgSg), though extremely
beautiful in hue, cannot be relied on as a pigment. Even
in oil or varnish its colour fades : Sir Joshua Reynolds'
experimental canvas shows some pale brown patches
which have once been kings' yellow, but which now have
almost entirely disappeared. Strange to say, in one of
his trials, a few quite visible crystals of orpiment are
preserved. As it cannot be imagined that he used this
pigment in this exceedingly coarse form, it would seem
that a molecular aggregation of a part of the orpiment
has taken place in the lapse of years. If this change has
not occurred, then we may conclude that only the largest
particles of the kings' yellow have escaped alteration.
Under any circumstances the inadmissibility of kings'
yellow to the palette of the artist is obvious : moreover,
it cannot be safely mixed with any pigment containing
lead or copper. It was known to the Egyptians.
Another compound of arsenic and sulphur (rVsgSg) has
been employed as a pigment. It is of an orange-red hue
and is known as realgar. Not only is it extremely poison-
ous, but it sufi'ers change on exposure to light, and acts
injuriously upon colours containing copper or lead. It was
used by the Romans : I identified a fragment of it amongst
the objects discovered at Silchester in the year 1896.
PURE ORANGE 185
Pure Orange : Marigold — Alizarin Yellow — Alizarin
Orange.
Under the above names a pigment of great richness
and beauty has been introduced. It is a kind of lake,
and consists of a coal-tar colour known to chemists as
/3-nitro-alizarin thrown down upon an aluminous base.
Nitro-alizarin, as its name implies, is a derivative of
alizarin, one of the least changeable of all organic pig-
ments, and the chief tinctorial product of madder. There
is no doubt about the beautiful hue of this paint, a deep
brownish gold : the trials first made as to its permanence
in oil promised well, but on continuing the exposure to
light of this pigment ground in oil it became evident that
it suffered considerably. The deterioration was more
marked when the paint had been mixed with flake white,
but even as a glazing colour it is not safe.
CHAPTER XV
RED PIGMENTS
Vermilion : Cinnabar — Vermilion — Zinnohev.
The mineral cinnabar, or mercuric sulphide, occurs in
many parts of Europe, and abundantly in China, and is
extensively worked in New Almaden in California ; it
would be tedious to recount the numerous localities in
which it has been, or is, found. Its colour in the mass
varies from cochineal-red and red-brown to lead-grey ; its
powder is usually scarlet, or red. Its hardness lies
between that of gypsum and that of calc-spar. It seldom
contains even i part in loo of impurities, but consists in
loo parts of very nearly 14 parts of sulphur by weight,
united with 86 of mercury, or i atom of each element.
The density of native vermilion is about 9. Vermilion
was formerly known as vermiculus, cinnabaris, ceno-
brium, and minium ; the last name is now appropriated
to red lead. Vermilion and vermiculus are derived from
the Latin vermes^ a name originally designating the
'kermes' insect found on the ilex or evergreen oak, which
is still used for the preparation of a red dye. From
kermes, in its turn, the words crimson and carmine are
derived. The name cinnabar is supposed to be of
Oriental origin (compare the Persian zanjifrak), and was
used sometimes to designate dragon's blood, a red resin.
186
VERMILION 187
Theophrastus informs us that two kinds of cinnabar were
known to the Greeks. One of these was undoubtedly
real cinnabar (chiefly from Spain), the other was red
lead. PHny's * cinnabar ' or ' minium ' was true ver-
milion, so was the * minium ' of Vitruvius. Theophilus
calls it 'cenobrium,' Wyclif 'cynoper,' Hakluyt * cinaper,'
and Ben Jonson * cinoper.'
One of the most curious facts concerning vermilion is
that it is identical in the nature and proportion of its
two constituent elements with an artificial black sub-
stance, ' ^thiop's mineral/ The red substance may be
changed into the black, and vice versa, and this without
any loss or gain, or any alteration of chemical com-
position, the change being a physical or molecular one
merely. The black substance is amorphous, the red
crystalline.
The pigment vermilion may be made by simply grind-
ing selected pieces of native cinnabar, or it may be
obtained artificially by combining the two elements
sulphur and mercury.
All the methods of preparing vermilion artificially may
be grouped under two divisions. The first of these is the
dry way, the other the wet way. In the former method
metallic mercury 42 parts, and sulphur 8 parts, are inti-
mately mixed and agitated together in revolving drums
until they have combined. The brownish-black powder
thus obtained is then submitted to sublimation in vertical
iron cylinders, surmounted by heads which are connected
with receivers. On sufficient heating, the mercuric sul-
phide sublimes as cinnabar or vermilion, the best part
condensing in the retort-heads. The rest of the sublimed
product (which has travelled farther) contains free sul-
phur, and is of inferior colour. The selected portions are
1 88 VERMILION
next ground, moistened with water, warmed with a Httle
caustic potash solution or nitric acid, and then thoroughly-
washed with boiling water. In another dry process the
mercury is gradually added to the proper proportion of
melted sulphur in an iron basin. When the combination
(which is accompanied by a violent evolution of light and
heat) is complete, the fused blackish mass is poured out,
broken into fragments, heated until excess of sulphur has
been driven off, and then sublimed in the way already
described. Some makers add to the crude sulphide, pre-
vious to sublimation, i per cent, of antimony sulphide,
with the object of improving the colour ; the product is
afterwards ground, digested with liver of sulphur, and
then washed with hydrochloric acid.
There are numberless processes for preparing vermilion
by the wet way. One of the best of these consists in
grinding, in the presence of water, lOo parts of mercury
with 38 parts of flowers of sulphur until these elements
have united. The black product is then triturated at
45° C. for many hours with a solution of 25 parts of
caustic potash in 150 parts of water. When the product
has attained its maximum of redness and beauty, it is
thrown into water, and thoroughly washed by decantation.
In a second process mercury, sulphur, and potassium
pentasulphide are boiled together for three or four hours,
and then the mixture is kept at a temperature of 50° C.
for several days. Vermilion may also be prepared from
the black sulphide obtained by precipitating a mercuric
salt with a soluble sulphide, from ' white precipitate,' and
from metallic mercury itself, by warming any one of these
substances with a solution of an alkaline pentasulphide,
and then purifying the product by means of a potash-
solution heated to 45° C. It has also been found that ver-
VERMILION 189
milion is produced when a mixture of mercurous chloride
(calomel) and zinc sulphate is heated to 45° — 50° C. with
an excess of a solution of sodium thiosulphate.
Except where carmine or realgar (red sulphide of
arsenic) is present, a very simple test suffices to ascertain
whether vermilion be pure or not. A small pinch should
be heated over a spirit-lamp on a fragment of hard porce-
lain ; no appreciable residue will be found, unless red-
lead, red iron oxide, brickdust, or other non-volatile
adulterants be present. Carmine, which is sometimes
added to scarlet vermilions to approximate their hue to
that of the crimson varieties such as the Chinese, may be
detected by laying a pinch of the powdered pigment on a
small pad of white blotting-paper, and moistening the
substance with a few drops of strong ammonia-water ; a
crimson stain will appear on the paper if carmine or
crimson-lake be present. The colour of a good vermiHon
is not changed by moistening it with nitric acid. The
accidental impurities which impair the hue of vermilion
are free sulphur, and compounds of iron and lead ; that
prepared in the wet way often retains alkaline salts,
owing to imperfect washing. A spurious vermilion,
called anti-vermilion or antimony vermilion, is made by
warming antimonious chloride with sodium thiosulphate
solution. It is the chief material used in colouring red
rubber.
Vermilion prepared from the mineral or native cinnabar
is probably less liable to change than the artificial pro-
ducts, whether obtained by the dry way or the moist way ;
but • moist way ' vermilions are certainly the most alter-
able. And it may also be remarked that the more finely
a vermilion is ground, the less stable it is — at least, as a
water-colour paint. Thus it happens that, other things
igo VERMILION
being equal, an orange-vermilion is inferior in perma-
nence to a scarlet, and a scarlet-vermilion to one inclining
to crimson. As an oil-pigment, vermilion does not dry
well, but suffers, especially if it be locked up in copal or
paraffin, no change by light or impure air ; loo parts of
the dry substance require less than 20 parts of oil. Owing
to its great density, vermilion tends to separate from the
oil with which it has been ground. This result may be
obviated by the addition to the oil of a little aluminium
oleate or linoleate, or by the employment of oxidized and
thickened oil in which a small quantity of beeswax or
ceresin has been dissolved by the aid of heat. In water-
colour painting most vermilions are found to be changed
on exposure, the solar rays gradually converting the red
into the black modification of mercuric sulphide, without,
of course, producing any chemical alteration. This
change occurs even in the absence of air and of moisture.
Impure air, per se, even if sulphuretted hydrogen be
present, does not discolour vermilion.
Anyone who has examined old illuminated manuscripts
must have noticed the apparent capriciousness with which
the ornaments, and especially the initial letters, painted
with vermilion, have been afifected. I have more than
once observed that, while all the vermilion used in one
part of a missal or choral-book has remained red, a leaden
hue has spread irregularly over the rest of the work in
places where this pigment has been used. This may be
due to the use by the illuminator of a sample of vermilion
adulterated with minium or red lead, but sometimes to a
change in the technique, as a change in the style or handi-
work is often associated with the difference above de-
scribed. In oil-painting there are no permanent pigments,
save the copper-greens, with which vermilion may not be
VERMILION 191
safely mixed. Only when it contains impurities, such as
free sulphur, does it darken flake-white.
Vermilion prepared from native cinnabar is found per-
fectly preserved in the flesh-tints of Italian tempera-
paintings of the thirteenth and fourteenth and fifteenth
centuries. It has stood in the wall-paintings of Pompeii,
where it often seems to have been waxed. A compara-
tively recent but instructive instance of the permanence
of vermilion in oil is furnished by a portrait, dated 1758,
in the National Portrait Gallery. It represents the painter,
Hogarth, with his palette set before him. The second of
the dabs of colour thereon is vermilion, perfectly intact.
In the same collection there is a portrait by Marc Ghee-
raedts of Mary Sidney, Countess of Pembroke, in which
the vermilion has stood. This work was painted in 16 14.
Scores of earlier and later examples might be cited.
The variations in hue observable in different specimens
of vermilion are mainly due to the differing degrees of
fineness in which the pigment occurs. The coarsest grain
corresponds with a crimson hue, and then we have every
variety of colour ranging from scarlet to reddish orange
or orange. The processes of regrinding and 'washing-
over ' enable us to obtain the kinds separately. And if we
repeat these operations often enough, we may ultimately
convert the whole of a crimson vermilion into the orange
form. It was formerly supposed that the latter material
was a mere scum, or impurity, or at least differed from the
crimson kind in composition. When any vermilion is
mixed in tint with white, an opposite effect to that of
further grinding is produced. For, as the early writer
Eraclius states : ' If you mix white with vermiculus, car-
mine is made ' — that is, the hue of the mixture becomes
more rosy, and therefore further removed from orange.
192 MADDER PIGMENTS
Madder : Pink Madder — Rose Madder — Madder Carmine
— Madder Red — Rubens' Madder — Madder Purple —
Madder Lake — Madder Brown — Carmin de Garance —
Laqtie de Garance — Krapplack.
Some authorities assert that madder was used in dyeing
long before its employment in painting. But there is some
evidence, derived from ' finds ' of pigments and from paint-
ings, that the ancient Greeks and Romans were acquainted
with a pink pigment derived from madder, while there
are good reasons for believing that such substances were
widely known in Europe as early as the thirteenth century.
Even in England, such a pigment is almost certainly re-
ferred to, under the name ' sinopis,' in the middle of the
fourteenth century. Now Alcherius (close of fourteenth
century) tells us that ' sinopis is a colour redder than ver-
milion, and it is made from varancia.' * Varancia ' is
clearly garance — that is, madder — the same material being
named ' warancia ' and ' waranz ' in a British Museum
manuscript (Sloane, No. 416) which contains recipes of
the fourteenth century. Besides ' sinopis ' (strictly, a red
earth), madder-lake was called, in English account-rolls of
the fourteenth century, * sinopre ' and * cynople.' It is,
however, difficult, if not impossible, to ascertain the pre-
cise date at which pigments derived from madder came
into use in the various schools of painting in Europe. For
the nomenclature of pigments has always been somewhat
vague, while the evidence furnished by existing pictures
does not at present enable us to trace back with absolute
certainty the mediaeval use of madder paints to an earlier
time than the fifteenth century. Eraclius does not men-
tion madder, nor does Cennini, who lived at a much later
time. Mr. R. Hendrie, in his notes to ' Theophilus,'
MADDER 193
speaks of an English manuscript of the fourteenth century
in which directions are given for extracting the colouring
matter of ' madyr.' From these directions we are, per-
haps, justified in concluding that the preparation of a kind
of liquid paint was intended.
The European madder-plant, a native of Greece, belongs
to the tribe Galiese, of the order Rubiaceae ; it is the Ruhia
tinctomm of Linnaeus, Several other species of this genus
are used or grown in India for the sake of the red dye they
afford. Among such species, Riihia covdifolia (Linn.) and
R. sikkimensis (Kurz.) may be named, but the European
madder is also cultivated extensively in India. Much
madder was formerly grown in the Levant, in Holland,
and in the south of France ; but the manufacture by arti-
ficial means from the anthracene of coal-tar of its two chief
colouring matters, alizarin and purpurin, has almost en-
tirely extinguished the cultivation of the madder-plant in
Europe. We shall have something to say presently con-
cerning the artificial products above named.
The root of madder contains a much larger proportion
of the colouring matters (or, it would be more correct to
say, colour-making substances) than the other parts of
the plant. They occur dissolved in the yellow cell-con-
tents of the soft tissue of the root. The finest madder
was grown in the ' Palud,' a chalky valley near Vaucluse.
But the cultivation of this plant was carried out in great
perfection in Zeeland during the eighteenth century.
The colouring matters obtained from madder exist in
the plant in the form of glucosides. These glucosides are
resolved by the fermentation, brought about by a peculiar
ferment in the plant itself, and by many chemical agents,
such as mineral alkalies and acids, mainly into glucose on
the one hand, and on the other into the several colouring
13
194 MADDER LAKES
principles. Of such colouring principles the glucosides in
madder yield at least three, of which the most important
are these two :
1. Alizarin, C^^HgO^.
2. Purpurin, Cj^^HgOg.
Both alizarin and purpurin are now manufactured arti-
ficially from anthracene. This compound, which occurs in
coal-tar, is a crystalline fluorescent hydrocarbon, C^^H^q.
By a series of processes this substance gives rise to alizarin
and purpurin, which are in all respects identical with these
colouring matters as derived from, the madder plant itself.
The artificial alizarin of commerce contains several other
colouring matters, two of which are better known than the
others ; these are anthrapurpurin (C^^HgOg) and purpuro-
xanthin (Cj^^HgO^). Purpuroxanthin is also present in the
natural pigments derived from madder, but it exists in
small proportion. Of all these compounds alizarin is the
most important and the best known, and yields lakes
having various hues of crimson, rose, purple, violet and
marone, according to its purity, its concentration, and the
nature of the base (alumina, aluminium phosphate, iron
oxide, manganese oxide, copper oxide, or lime with
alumina) with which it is associated. The purpurin and
anthrapurpurin resemble one another closely, and give
pigments which are generally characterized by more
orange or red hues than those obtained with alizarin.
The rose and pink madders and the madder carmines of
commerce are generally so manufactured as to include,
for their colouring constituents, much alizarin and very
little purpurin. A few indications of the ordinary methods
of preparing these lakes may first be given.
The material used is often that called * madder flowers,'
which consists of the finely ground dried root after it has
MADDER LAKES 195
been submitted to the action of dilute sulphuric acid and
washed. Four pounds of this madder are taken and
warmed for two or three hours on a steamer, with a solu-
tion of I pound of pure alum in i gallon of water. The
mixture is placed in a filter-press, and the liquor obtained
(which must be perfectly clear) precipitated by the gradual
addition of a solution of sodium carbonate. The first
portions of madder lake which fall, being the best, should
be collected apart. All the precipitates should be
thoroughly washed with rain or distilled water till the
wash-waters are no longer troubled on the addition of
barium chloride solution; they are then moulded into small
cones, drops, or discs, and carefully dried at a moderate
temperature. Another process for preparing madder lakes
is a modification of the above. Four pounds of madder-
root in powder, after having been fermented and then
washed with a weak solution of sodium sulphate, are
boiled for fifteen minutes with 4 gallons of a 10 per cent,
solution of pure alum, the whole is filtered, and at a tem-
perature of 45° partially neutralized with a solution in
water of about 8 ounces of pure sodium carbonate. The
liquor is now brought nearly to the boiling-point ; the
madder lake which is then deposited is to be thoroughly
washed and then dried : it is much denser than that pro-
duced by the preceding process. In the manufacture of
alizarin lakes it is customary to introduce a small quantity
of a preparation known as Turkey-red oil or sulphated
castor oil. This is made into a soap and added to the
alkaline solution employed to precipitate the lake.
By the employment in various proportions of solutions
of alum and calcium chloride, by the substitution of
sodium phosphate for the carbonate, and by choosing
various qualities of madder-root, a number of hues and
196 MADDER LAKES
tints of rose and pink madder may be obtained when one
or other of the methods above described is adopted. The
oxides of iron, manganese and copper, when used in asso-
ciation with more or less alumina as a base for receiving
the various colouring matters of madder, give other hues,
including madder purple and madder brown.
But occasionally the pigments sold under these names
are mixtures. For instance, burnt sienna and copper f erro-
cyanide have been found in samples of madder brown; the
presence of copper in madder brown seems, however, to
be usual, but it arises from the employment of copper
sulphate in its preparation along with alum.
From alizarin and from purpurin (either natural or
artificial) lakes may be readily prepared by dissolving
these substances in the smallest necessary quantity of an
alkali, such as ammonia or sodium carbonate, and then
adding a solution of a pure aluminium salt or some pure
freshly precipitated and thoroughly washed aluminium
hydrate. Another and more recent process consists in
dissolving the colouring matter in a solution of sodium
aluminate, and then precipitating the * lake ' by adding
dilute sulphuric acid or, better, a solution of alum.
The best artificial alizarin of commerce occurs as a
yellowish powder, presenting the aspect of raw sienna. It
may, however, be obtained in yellow or orange red crystals,
either by repeated crystallization from a solvent or by
sublimation in vacuo. Its colour is always brighter than
that of purpurin, which in powder has about the hue of
Venetian red. But when solutions are made of these two
substances in alkalies, then it is seen that the colours are
reversed — alizarin yielding a crimson verging upon purple,
and purpurin a red verging upon crimson. Differences of
colour will be noticed in the lakes prepared with these two
ALIZARIN AND PURPURIN 197
bodies. The directions for preparing pigments from the
above-named bodies are practically identical with those
already given in outline, but the minute details of manip-
ulation can be learned only in actual practice. The
following process gives an artificial red madder of excel-
lent hue : Equal weights of pure alum (absolutely free
from iron and lime) and of the purest artificial purpurin
in powder are ground together, and then washed with cold
water until the washings are colourless ; then the residue
on the filter is boiled with a 5 per cent, solution of pure
alum, filtered while boiling, and immediately neutralized
with pure sodium carbonate solution (also boiling) until
red flocks appear. These are filtered off, and constitute,
when washed and dried, a fine pigment of a rich red hue.
By heating the mother liquor to 80°, and adding more
sodium carbonate, a further and equally good product is
obtained. The purpurin residue, when again heated with
more alum-solution and precipitated as above directed,
yields a further quantity. The final residue, after several
such exhaustions, produces an impure lake, having a
brownish-red hue. A very large number of commercial
preparations of alizarin and of other dye-stuffs closely allied
to it are now available for the preparation of the so-called
* madder lakes.' Some of these preparations when dis-
solved and then precipitated on a suitable basis, yield pig-
ments of great richness and stability, others, especially
those which possess a yellow, orange, or red-brown hue,
are less permanent. Here it may be remarked that,
broadly speaking, the true or * root ' madders are complex
so far as their colour-constituents are concerned and
simple as regards their base, while the converse is true
of the alizarin lakes.
Although the madder colours are very much less aft'ected
198
MADDER LAKES
by light than are the pigments derived from cochineal, yet
it cannot be affirmed that any of them are absolutely per-
manent when continuously exposed. The following figures
show approximately the amount and nature of the change,
observed after certain intervals, in the case of several
madder pigments used as water-colours :
Name of Pigment
Original
Intensity = lo
Change of Hue
♦Madder Carmine, A
Afte
r I ;
y^ear,
10 -
Very slight.
>> )>
B
I
8 -
Much more purplish.
" j>
C
5
2 -
»> j>
C
7
O -
*
F
4
lO -
More purplish.
Madder Red -
- .
I
6 -
Less red, more blue.
Rose Madder -
- -
I
8 -
Slightly more purplish.
„ B
2
3 -
„ B
5
I -
Smoke grey.
B
7
I -
Grey.
*.. „ F
4
H ■
Slightly more purplish.
Pink Madder -
2
I -
Purple Madder,
A -
I
7 -
Duller, less red, more
5» >J
C -
2
6 -
More bluish. [blue.
/>
D -
5
7 -
fJ ..
C -
7
2 -
»> 11
E -
7
9 -
Somewhat puce.
♦Brown Madder, A
I
9 -
Less red, more yellow.
j> »i
B
2
I -
Grey.
j> » J
B
5
I -
>> )>
B
7
o -
Grey.
The letters A to F indicate different samples of the several
pigments, which were in all cases ' moist ' colours ; a
parallel but less complete series with * cake ' colours gave
practically the same results. The five samples marked *
are instances of exceptional stability, and are of import-
ance as showing the possibility of obtaining some, at all
events, of the madder pigments in a satisfactory form.
It is noticeable that the paler (pink and rose) madders,
MINERAL LAKE i99
which contain much water, are generally more perishable
than the concentrated madder carmine ; the comparative
trials having, of course, been made with washes of nearly
the same depth of tint.
A study of this table inclines one to think that the
genuineness and purity of some of these pigments are
doubtful, yet one specimen only (Madder Carmine C) was
not tested. In this case the material used was not available
for analysis, but I have no reason to doubt its authenticity.
Mineral Lake : Pink-Colotiv — Potters' Pink — Laque
Minerale — Minerallack.
Attempts have been made to obtain mineral pigments of
absolute permanence in order to acquire substitutes for
the reds and purples of vegetable origin. None of them
equals in intensity and splendour of colour the derivatives
of madder. One of the best of these substitutes is mineral
lake. This compound may be made in many ways. In
some recipes stannic oxide, chalk and a little potassium
chromate are directed to be heated together : in one pro-
cess the operations are begun by precipitating a solution
of neutral potassium chromate by means of a solution of
stannic chloride. The precipitate is collected on a filter
and thoroughly washed. Still moist, it is ground into a
paste with half its bulk of pure nitre and some stannic
oxide, and allowed to dry. The dry mixture is projected,
little by little, into some nitre heated to low redness in a
crucible. When the basic chromate of tin has settled, the
nitre, still fused, is poured off. and the residue washed
thoroughly with water. The product thus obtained
requires calcination for two hours at a high tempera-
ture in a luted crucible, in order to develop its colour,
which much resembles that of almond-blossom when
200 LIGHT RED
the matter is finely ground. It appears much richer in
hue and less opaque when used as an oil-colour. It
constitutes an unalterable pigment.
Under the name of Potters' Pink Mr. W. Burton, of
Pilkington's Tile Works, has introduced to the notice of
artists a ceramic pigment which is a variety of that which
has been just described. He says of it that it was
* invented in Staffordshire by an unknown potter about
a hundred years ago. It is obtained by calcining a
mixture of oxide of tin and lime with a mere trace of
oxide of chromium. It is a semi-opaque colour, unlike
any usually supplied to artists. It should be particularly
valuable in paintings in which it is undesirable to use
madder or alizarin pigments.' It is scarcely necessary
to add to this account that Potters' Pink is not only a
permanent pigment which may be used in all methods of
painting (including fresco), but that it is without action
on other pigments.
Light Red: Btiynt Ochre — Rouge Anglais — Brun Rouge
— Englischvot.
Light red is, or ought to be, yellow ochre burnt — that is,
calcined. The different varieties of yellow ochre yield, as
might be expected, products having various hues and
tints of this rather pale and dull brownish or orange red.
Moreover, these hues depend in some measure upon the
temperature at which the calcination is effected. To
prepare light red, the selected yellow ochre is usually
crushed and then roasted on an iron plate heated to red-
ness. When the desired tint has been attained the
material is thrown into cold water, ground, and washed.
Light red may also be made by conducting the finely-
divided yellow ochre suspended in a current of air into a
VENETIAN RED 201
heated chamber or furnace. Light red consists, then, of
yellow ochre deprived of its water of hydration by means
of heat. It is necessary to employ yellow ochre as free
as possible from organic matter and from lime if a bright-
coloured product be desired.
Light red possesses a considerable degree of opacity.
Its hue may be defined as a scarlet, modified by a
little yellow and grey. It is perfectly permanent and
without action upon other pigments.
Light red boiled with hydrochloric acid will, if genuine,
yield a solution, which after filtration will give no pre-
cipitate, but merely a slight cloudiness, on the addition
of a few drops of barium chloride solution. The terms
' rouge Anglais ' and ' Brun rouge ' are not infrequently
applied to artificially prepared iron reds.
Venetian Red : Rouge — Crocus — Colcothar — Caput
Movtumn Vitvioli — Venetianischrot.
Originally Venetian red consisted of a native ferric
oxide or red haematite, less purplish in its tints and
washes than Indian red. But of recent years the name
appears to have been transferred to a particular quality of
artificial ferric oxide, made by calcining green vitriol.
When this salt is heated in a crucible the upper portion
of the product, which has been less strongly heated than
the lower, is of a brighter red than the remainder, and
after washing and grinding is sold as Venetian red. If
moistened with a solution of nitre, again heated, and then
ground and washed, the red tint of the product becomes
somewhat brighter.
The hue of Venetian red is less brownish than that of
light red, and not at all purplish like that of Indian red.
Venetian red, whether artificial or natural, is a per-
202 INDIAN RED
manent pigment which may be mixed with other permanent
pigments without fear of injuring them ; but it must be
perfectly free from soluble salts and from any trace of
sulphates. The presence of the latter may be detected by
the test described under ' Light Red ' and * Indian Red.'
But few commercial samples will stand this test, however,
and we consequently find that many samples of Venetian
red, owing to the presence therein of sulphates, exert an
injurious action upon some of the organic pigments used
as water-colours — notably, upon indigo.
A very fine native red ochre comes from Tuscany and
from Krasso in the Banat, Hungary. It is represented
by the formula 2Fe203, HgO, and goes under the minera-
logical name of tuvgite. Its hue is that of a fine Venetian
red : it is probable that the fine native Indian reds and
red ochres sometimes consist of or contain this hydrated
ferric oxide, and are not really anhydrous.
Indian Red : Persian Red — Indian Red Ochre — Indischrot.
Indian red is a variety of red ochre, or red haematite,
containing about 95 per cent, of ferric oxide, and having a
slightly purplish hue. It varies somewhat in quality, and
often requires sifting through a fine silk sieve, followed by
washing over, in order to fit it for use as a pigment. Most
of the Indian red imported from India is a natural pro-
duct, but some has been prepared by calcination. Some so-
called Indian red is imported from Ormuz in the Persian
Gulf; some is an English haematite from the Forest of
Dean.
A recent recipe for making an artificial Indian red
directs that a mixture of 75 parts of green vitriol be taken
and dried at a moderate heat previous to mixing it wdth
18 parts of calcined magnesia and 7 parts of common
INDIAN RED 203
salt, all the ingredients being in fine powder. The mix-
ture is then ignited, preferably under reduced pressure,
and the residuum thoroughly washed with water. By the
introduction into the original mixture of a little aluminium
sulphate the purple hue of the product may be enhanced ;
indeed, it is quite possible in this way to obtain the pig-
ment known as Mars violet.
Indian red, when genuine, is a perfectly permanent pig-
ment in all media, and is without action upon other colours.
It was extensively employed by the older masters of the
English Water-Colour School, in association with true
ultramarine, with Prussian blue, with indigo, or with indigo
and yellow ochre, to produce the lilac greys of stormy
clouds. The indigo in some of these greys having often
perished, the Indian red (and the yellow ochre where
employed) remains intact, giving a hot and frequently
foxy red to spaces which were originally cool in hue, and
comparatively neutral. This change has been incorrectly
attributed to an action exerted upon the indigo by the
Indian red. But as indigo disappears when used alone, or
when a thin wash of it on a sheet of gelatine is placed
over, but not in contact with, a wash of Indian red, the
current explanation of the phenomenon in question cannot
be true. Greys made with light red or Venetian red show
similar alterations of colour. Colcothar, or jewellers' rouge,
the red oxide of iron obtained as a residue when green
vitriol (ferrous sulphate) is calcined, has sometimes been
called Indian red, and substituted for the native oxide.
Those portions of the above-named residue which have
been more strongly heated generally present something of
the purplish red hue which belongs to the true native
Indian red. And this peculiar hue may be imparted to
ordinary rouge by moistening it with a weak solution of
204 RED OCHRE
potassium chlorate, drying, and then calcining the mass
once more. It generally contains basic ferric sulphate,
and then should be looked upon with suspicion, for it may
seriously damage the indigo and other organic pigments
with which it is associated. If a small pinch of Indian
red be boiled with hydrochloric acid, thrown on a filter,
and the filtrate tested with barium chloride solution, the
genuineness of the pigment wdll be proved by the absence
of any white precipitate of barium sulphate.
Red Ochre : Red Hmnatite — Red Iron Ore — Scarlet Ochre
— Red Chalk — Ruddle — Bole — Sinoper — Sittopis — Ruhrica
— Miltos — Terra Rosa — Arvahida Red — Bolus.
The pigments above-named are native ferric oxide (or
iron peroxide) associated with variable proportions of
mineral impurities such as clay, chalk, and silica. They
differ from the yellow and brown ochres described on
page 157, by not containing combined water, in other
words, the iron to which they owe their colour is ferric
oxide, not ferric hydrate, except in the case of turgite,
named on page 202. They occur in very many localities
accompanying or even constituting some of the most im-
portant iron ores. Their colour varies with their physical
state, and with their purity ; some are iron grey, or even
black, until they are finely ground, when they assume a
cherry-red hue. Cappadocia yielded to the ancient Greeks
some at least of their sinopis, or red ochre, but they were
famihar with the process of calcining yellow ochre in
order to redden its hue ; and they thus prepared the pig-
ment to which the name * light red ' is now assigned.
An unusually bright red variety of haematite from
Cumberland gave me on analysis, in 100 parts, ferric
oxide, 947; alumina, 2-0; silica, 2*2, and moisture, i-i.
INDIAN LAKE 205
But some almost equally rich red ochres contain much
less iron oxide, a * sinopis ' from Anatolia, analysed by
Klaproth, having been found to contain 21 per cent,
only, and others, from other localities, not above 40.
The paler varieties of a reddish brown or yellow brown
hue often consist of mixtures of haematite and yellow
ochre. A very fine red ochre from Tuscany, the Banat,
Hungary, and from the Urals, contains about 5 per cent.
of water, and is considered to belong to a distinct mineral
species called kirgite, as mentioned before under the
heading ' Venetian Red.'
When red chalk (from Hunstanton, Norfolk) is calcined
at a high temperature it loses its red colour, and becomes
of a dull olive green hue, a change due in this case to
the production of calcium ferrite, a compound of lime
and ferric oxide.
The terra rosa of Italy owes its pinkish red hue to
ferric oxide, but it is probably often, if not always, an
artificial product.
All the pigments described under the title ' red ochre '
are permanent, and without action on other pigments.
For the substitution of artificially prepared ferric
oxide, or colcothar, for red ochre, and for the method of
detecting it, reference should be made to page 202, under
* Indian red.'
Indian Lake : Lac Lake — Lack-Lack.
Lac is a resinous secretion produced by certain plants
when punctured by the larvae of the Coccus lacca, an
East Indian hemipterous insect. Amongst the trees
which the insect chiefly attacks are Butea frondosa, Ficus
religiosa, and F. hengalensis, Schleichera tvijuga, Shorea
robusta, and Ztzyphus jujuha. The lac, though a secretion
2o6 INDIAN LAKE
primarily derived from the tree on which the insects
feed, is yet profoundly modified, particularly with respect
to its colour, during its passage through the animal's
body. It varies in colour with the species of tree, but
always consists mainly of three substances— namely, a
resin, a colouring matter, and a wax. The resin, which
constitutes two-thirds of the substance, is obtained in the
form known as ' seed-lac ' by pounding in water the lac
which has been removed by pressing with a roller the
encrusted twigs on a floor. The water becomes red ;
from it, by evaporation, the crude < lac-dye ' is obtained.
This is made into cakes, and dried.
The lac-dye of commerce contains nitrogenous and
mineral matters, as well as several dark-coloured
impurities, and some resin. In order to prepare a lake
from it, it should be first powdered, and then digested in
spirit of turpentine, or benzene. The purified residue
is, when dry, extracted with sodium carbonate solution :
the liquor is afterwards filtered and precipitated with
alum solution. The precipitate thus formed is washed
and dried in the dark.
Indian lake was used by the Venetian and Flemish
painters of the sixteenth century, but it does not seem,
so far as one can gather from the notices of it in the
work of De Mayerne, and in the Secreet-Boeck, to have
been often obtained of good colour. It is even spoken
of as a ' light brown.'
Indian lake is inferior in beauty, and in variety of
hues, to the colours from madder ; it is also more affected
by light. But it is distinctly less fugitive than crimson
lake and the other cochineal pigments.
I do not know from direct experiment whether the com-
parative stability of the red colouring matter from kermes
COCHINEAL LAKES 207
when used as a dye for animal fibres belongs also to all
the lakes prepared from this substance : anyhow, kermes
pigments are, so far as I can learn, not met with in com-
merce at the present time. Their use in European paint-
ing seems to have been displaced, first of all by Indian
lac lakes, and then by cochineal lakes. The colouring
matters produced by these three kinds of coccus are
closely allied chemically.
Carmine and the Cochineal Lakes : Carmin—Laque
Cramoisie, or Crimson Lake — Purple Lake, etc. — Karmin.
Cochineal consists of the dried wingless females of a
species of coccus (C. cacti) which feeds upon several kinds
of Opimtia, or cactus. The best quality comes from Tene-
riffe, and contains about half its weight of colouring
matter. This colouring matter is a glucoside, to which
the name of carminic acid has been given. When this
substance is treated with weak sulphuric acid, it is
resolved into a sugar and another colouring matter called
carmine red.
The carmine of commerce is prepared directly from
cochineal, and is the most concentrated and purest form of
any of the pigments derived from this source. The
methods of preparation differ, but in all the colouring
matter is extracted from the insects by means of boiling
water, with the subsequent addition of small quantities of
alum, or nitre, or potassium oxalate, or cream of tartar ;
occasionally a small quantity of stannous chloride is em-
ployed also. The liquor, after a repose of some days or
even weeks, deposits a great part of the colouring matter
as a deep crimson-red powder, which is then thrown on a
filter, washed and dried in the dark. However prepared,
it contains fat, albuminoid matter, mineral salts, and
2o8 COCHINEAL LAKES
other impurities ; the finest varieties, however, dissolve
perfectly in strong liquor ammonise. The liquor, which
has deposited the carmine, gives up the remainder of its
colouring matter to freshly - precipitated aluminium
hydrate, or, after having been rendered alkaline by
potassium carbonate, to a solution of alum. Lakes are
thus formed. Lakes are also made by directly precipi-
tating cochineal extract with solutions of potassium car-
bonate and alum. A purplish tinge is given to the product
by a small quantity of lime ; other hues, generally dull,
are imparted by the presence of iron, manganese, or
copper in the solutions employed. The cochineal lakes
always contain a larger quantity of water and of alumina
(or other mineral basis) than carmine, and are conse-
quently weaker. Scarlet lake is usually a mere mixture
of crimson lake and vermilion, but alizarin lake may
replace the former.
Beautiful and rich as are the colours prepared from
cochineal, not one of them should ever find a place upon
the palette of the artist. They all become brownish, and
ultimately almost disappear after a short exposure to
sunlight or the more prolonged attack of strong diffused
daylight. In six hours of sunshine a strong wash of fine
crimson lake on Whatman paper lost 8 per cent, of its
original intensity ; this was on April 12. The loss during
a second period of six hours' exposure was much less, but
after the lapse of four months less than 5 per cent, of the
original colour remained. In the case of carmine, from
one to two years was required for the complete oblitera-
tion of every trace of the original crimson from a deep
wash of this pigment. All the cochineal pigments be-
come somewhat brownish during the course of fading,
but ultimately, when all the red has disappeared, either
BURNT CARMINE 209
a greenish-grey or a faint sepia-like brown is the sole
residue.
The term 'lake' belongs to all colouring-matters
throv/n down upon such a basis as alumina ; but when
purple, crimson, or scarlet is prefixed to the word lake,
cochineal colours are always understood. So carmine used
alone refers to cochineal carmine, although it is a general
term for a group of rich pigments, of which madder car-
mine and indigo carmine are perfectly distinct examples,
derived from madder and indigo respectively.
It is not necessary to say more about the various
cochineal pigments, nor to point out their many falsifica-
tions, for their value as artists' colours is very small. No
artist who cares for his work and hopes for its permanency
should employ them.
Burnt Carmine.
This preparation should rather be called ' roasted car-
mine.' It is obtained by carefully heating the carmine
made from cochineal. It possesses a beautiful hue, but
is quite as fugitive as the product which yields it. Two
years' exposure to sunlight completely destroyed a strong
wash of cake-burnt carmine on paper. The moist pig-
ment had lost nine-tenths of its intensity at the end of
the same period, while the small residual proportion of
the colour had suffered no further change in depth at the
end of a further lapse of three years. Burnt carmine is
rather less fugitive in oil than in water colour. Experi-
ments in the latter medium gave, after exposure to
sunlight, the following residual intensities out of ten :
Moist, after two years ... ... i
Cake, after two years ... ... o
Moist, after five years ... ... i
14
210 RED LEAD
A sample of burnt carmine purchased of Messrs. New-
man about the year 1815 was found (as might have been
anticipated) to have retained its colour in the cake per-
fectly to the present year ; but a wash of it on paper
possessed no greater nor less degree of permanency than
a wash of the same pigment prepared by the same
makers in 1886.
Red Lead : Minium — Saturnine — Mijte Rouge, Mine
Orange — Mennige — Rosso Satimio .
This beautiful orange-red pigment approaches in com-
position a compound of two molecules of protoxide of lead
with one molecule of binoxide, and may be approximately
represented by the formula Vhfi^. The paler and more
orange-tinted varieties contain an excess of protoxide of
lead, often accompanied by a little carbonate.
This pigment is peculiarly liable to discoloration in the
presence of sulphuretted hydrogen : it acts energetically
upon some paints, on the cadmium yellows, for example.
It is quite inadmissible as a water-colour, and cannot be
considered as safe in oil.
Cobalt Red : Rose de Cobalt — Cobalt Violet — Kobaltrot.
This little-used pigment should consist of the oxides of
magnesium and cobalt. It is prepared at a high tempera-
ture and is quite permanent. One method of making
this pigment involves the use of magnesium carbonate or
oxide, which is made into a paste with a solution of pure
cobalt nitrate. This paste is then slowly dried, and ulti-
mately calcined in a crucible. Different preparations of
this pigment differ considerably in hue ; a purplish cast
is sometimes due to the accidental presence of alumina.
COBALT RED 211
Pigments consisting of cobalt arseniate are occasionally
called ' cobalt red ' ; the term ' cobalt violet ' is usually
and may be more fitly applied to them. The hue they
present is rather bluer (or less red) than that of the
flowers of the common foxglove. Cobalt violet has been
made from the mineral known as erythrite, or cobalt-
bloom, which has the formula CogAsgOg, SHgO ; an
artificial cobalt arseniate is also made by oxidizing cobalt
sulph-arsenide, which is first reduced to powder and then
roasted with twice its weight of potassium carbonate.
After further treatment, the final product obtained by
grinding and washing constitutes a pigment of a rather
coarse grain which does not work smoothly as a water-
colour, but has the advantage of complete stability in all
vehicles. A sample of this cobalt violet of good quality
was found to contain no water and to suffer no change
when heated to a red heat in the air ; along with cobalt
arseniate it contained some phosphate.
CHAPTER XVI
GREEN PIGMENTS
Terre Verte : Green Earth — Tevre de Verone — Gvilne
Erde — Terra Verde.
There are two rather indefinite minerals, probably not
really distinct — namely, glauconite and celadonite — which
furnish the raw material from which the artists' pigment,
generally known as terre verte, is prepared. The form,
or variety, celadonite is the rarer of the two : it is softer
than glauconite : both minerals are probably mixtures.
Green earth of very good quality is found at Bentonico,
to the north of Monte Baldo, near Verona, where it occurs
in cavities in an amygdaloid rock. The best samples
possess a deep olive-green colour ; inferior specimens are
celandine or apple-green. Green earth is obtained from
a large number of European and American localities, and
varies much in chemical composition. From its greenish
hue it has been assumed to consist chiefly of a ferrous
silicate — that is, a silicate of protoxide of iron, and in this
way it is represented in the older analyses. But more
exact analyses have shov/n that green earth contains but
a small part of its iron in the protoxide condition, and that
it is mainly a ferric silicate. A choice specimen of fine
hue from Monte Baldo gave the following results on
analysis, the numbers representing parts in one hundred:
TERRE VERTE 213
Water, given off at 100° C. 4*1
Lime -
-
-
- i-i
Water, given off at a red heat 42
1 Magnesia -
-
-
- 5-6
Ferric oxide (Fe.^O.^) - 20 3
Potash
-
-
- 6-4
Ferrous oxide (FeO) - - 26
Soda -
-
-
- 2-3
Alumina - - - - 17
Silica
-
- 517
Green earth is thus allied to the hornblendes, differing
mainly in the partial replacement of soda by potash and in
the presence of water. Being itself an alteration-product,
it is not likely to be amenable to further change, particu-
larly as the iron in it is for the most part fully oxidized.
Terre verte occurs in ancient Roman wall-paintings (the
prepared pigment was found in the ruins of Pompeii), and
was largely employed by the early artists of Italy in their
works in tempera, fresco, and oil. But amongst the green
pigments found in ancient wall-paintings in Rome and
Pompeii there occurs also a substance of a richer and
deeper hue than that possessed by terre verte. It was
made by grinding into fine powder a kind of green jasper ;
it has proved quite permanent.
Terre verte is prepared by carefully selecting the richest-
coloured and most uniform specimens of the mineral,
grinding them to fine powder, and washing the pulverized
material with rain-water ; it is then dried. Sometimes the
selected fragments are heated, and then quenched in very
dilute hydrochloric acid to remove ochre and other im-
purities ; the undissolved portion is then ground, washed
thoroughly, and dried. Most samples of terre verte are
found to be perfectly stable both in water-colour and oil-
painting. It is a semi-opaque or translucent pigment,
without much body in oil. It has no action on, nor is it
affected by, other permanent pigments. When used in
oil or tempera as a ground-colour or in the under-painting,
terre verte sometimes becomes more conspicuous in the
214 GREEN OXIDE OF CHROMIUM
course of time, owing partly to the deepening of its own
hue and partly to the increased translucency of the pig-
ments which have been laid over it. Some samples of
terra verte seem to be liable to become sHghtly rusty
when brought into contact with lime hydrate in true
fresco painting. This is probably due to the further
oxidation of some of the ferrous oxide they contain. Cal-
cined terre verte is known as Verona brown.
Terre verte is rarely adulterated. A pure sample is not
affected when drenched with liquor ammoniae, becoming
neither more bluish (presence of copper compounds), nor
more brownish (presence of Prussian blue). But although
adulterated terre verte is rarely met with I have found that
a well-known firm of artists' colourmen sell, or have sold,
a mixture with green ultramarine under the name of
terre verte. Tubes of oil-colour of this sort have been
submitted to me for analysis. This substitution is unfair
to the artist, who has a right to obtain the material he
demands. Green ultramarine freed from oil gives sul-
phuretted hydrogen when moistened with hydrochloric
acid, which has no such action on terre verte.
Green Oxide of Chromium: Chromium Sesquioxide —
Tnie Chrome Green — Opaque Oxide of Chromium — Vert
de Chrome — Griines Chromoxyd,
The so-called native oxide of chromium, or chrome
ochre, is a mere greenish clay, containing not more than
lo per cent, of chromium oxide. But another mineral
(from Okhansk, in Siberia) is of a deeper green, and
sometimes contains one-third of its weight of this oxide.
We are not aware that any attempt to employ these
minerals as pigments has ever been made.
The green oxide of chromium, CrjOg, varies in hue, in
GREEN OXIDE OF CHROMIUM 215
depth of colour, and in opacity according to the process
of its preparation. It may be made either in the ' dry '
way or the ' wet ' way. Perhaps the finest quality is that
produced by heating mercurous chromate in a retort till
the whole of the mercury has been distilled off. This is
an expensive process, but the product is excellent in colour
and body. Ammonium bichromate gently heated, in small
portions at a time, yields a dull-coloured but a useful
variety of this pigment ; but there is a similar, though more
economical process, yielding a better-coloured product.
This consists in gently heating together and then calcining
a mixture of 3 parts of neutral potassium chromate with
2 parts of ammonium chloride. The mass is to be
thoroughly washed with hot water, dried, and again ignited.
Two other processes for preparing this pigment in the dry
way may be commended. In both of them potassium
bichromate (free from iron) is used, but in the one case
sulphur, in the other starch, is employed, in order to effect
the reduction of the chromic salt to the condition of ses-
quioxide. The sulphur method yields at once a good pig-
ment, which needs nothing but a thorough washing, first
with very dilute sulphuric acid and then with water, fol-
lowed by grinding, to fit it for use, but a second calcination
is required in the starch process. In order to prepare
oxide of chromium by the wet way, a solution of an
alkaline chromate or bichromate is to be reduced by
sulphur or other reducing agents, or a sesquisalt of
chromium is precipitated by ammonia or a fixed alkali.
The hydrated sesquioxide is thrown down ; after having
been washed, this substance is gently calcined. The
hydrate itself, when air-dried, constitutes one of the forms
of ' transparent oxide of chromium.'
The ordinary or opaque oxide of chromium usually
2i6 VI RID I AN
occuis in the form of a greyish-green powder of consider-
able body. It is quite permanent under all conditions of
exposure and of commixture with other pigments, and is
available in all the processes of painting. Its tints with
flake-white, and the yellowish greens which it yields
with aureolin, are peculiarly valuable to the landscape-
painter.
Green oxide of chromium is rather imitated than adul-
terated. This pigment when genuine is not altered in
colour either by ammonium sulphide or caustic potash.
But under the name of chrome-green mixtures of Prussian
blue and lead chromate are constantly sold. These prep-
arations generally contain some barium sulphate, often a
considerable quantity of gypsum, and, occasionally, alu-
mina. The term 'chrome-green' cannot be justly applied
to these mixtures, which are greatly inferior in stability
to the pigments under discussion, and are all discoloured
by lime and alkalies.
ViRiDiAN : Emerald Oxide of Chromhim — Vert Pannetiev —
Vert de Guignet — Vert Emeraude — Mittler's Green —
Feuriges Chvomoxyd.
About the year 1838 Pannetier and Binet began to make
a beautiful chromium green by means of a secret method.
Many years afterwards M. Guignet discovered and
patented a process by which this admirable pigment
could be manufactured. About 8 parts of crystallized
boracic acid and 3 parts of potassium, bichromate are
thoroughly mixed and calcined. The mass obtained is
treated with cold water, washed by decantation, ground
wet, washed with hot water, and carefully dried. The
product is a hydrated chromium sesquioxide, in which a
variable amount of the boracic constituent is often re-
VIRIDIAN 217
tained. Viridian is, however, essentially a hydrated ses-
quioxide of chromium, having the formula CrgOg, 2H2O.
In the preparation of this pigment it often happens that
sufficient care is not taken to wash it perfectly free from
all soluble salts. I have known the metal tubes in which
this moist water-colour has been kept to be corroded by
these salts and to fall to pieces in a few months. The
presence of such saline matter is easily detected by mixing
the moist or cake colour with water and noting if coagu-
lation or curdling takes place.
It would be difficult to exaggerate the value of this ad-
dition to the artist's palette. The colour of viridian is a
very deep bluish green of great purity and transparency.
It furnishes, with aureolin on the one hand and with ultra-
marine on the other, an immense number of beautiful hues,
adapted to represent the colours of vegetation and of water.
It is quite unaffected by sunlight and sulphuretted hydro-
gen, and it has no action on other pigments. Moreover,
it may be safely used with all the different painting media,
and upon all kinds of painting-grounds.
Viridian is the name by which this pigment is perhaps
now best known in England. It is unfortunate that it
should be called ' Vert Emeraude ' in France, since it has
little in common with the poisonous emerald green of our
colourmen. This pigment is sometimes adulterated with
baryta yellow in order to modify its hue. This substance
may be readily detected by moistening the pigment with
dilute hydrochloric acid, when a yellow solution is ob-
tained.
Arnaudon's chrome green is of a somewhat opaque
green hue, rather like that of Schweinfurt ; it is a phos-
phate of chromium. A similar product is Mathieu-Plessy's
chrome green ; but under the name of ' chrome green,'
2i8 COBALT GREEN
'green cinnabar' and 'griiner Zinnober,' spurious pig-
ments are constantly sold, the commonest of them con-
sisting of mixtures of chrome yellow and Prussian blue.
They are worthless in comparison with viridian.
A clear and bright green called ' vernalis' is one of the
potters' pigments introduced to the notice of artists by
Mr. Wm. Burton. It is formed at a very high tempera-
ture and is perfectly permanent. It contains lime but
owes its colour to chromium.
Cobalt Green : Rinmann's Green — Vert de Cobalt — Vert
de Zinc — Kohaltgriin .
It has long been known that the oxide or a salt of zinc,
moistened with a solution of cobalt nitrate, and then
strongly heated before the blow-pipe, gives a porous mass
of a beautiful green hue. This compound or mixture of
the oxides of zinc and cobalt may be prepared by : (i) Pre-
cipitating with an alkaline carbonate a mixture of the
nitrates of cobalt and of zinc, and then strongly heating
(after washing) the precipitate formed ; (2) Making a paste
of zinc oxide and water, and adding to it a solution of
nitrate or sulphate of cobalt, or of roseo-cobaltic chloride ;
the mass is then dried, calcined at a dull red heat, thrown
into water, ground, washed, and dried. Method No. 2
gives a finely coloured product, the depth of hue being
proportional to the percentage of cobalt oxide. If the
latter oxide amount to one-third of that of zinc the colour
is a very deep bluish green ; with no more than one-sixth
the colour is still rich. Some specimens do not contain
more than one-twentieth — sometimes even less — of cobalt
oxide, and yet they are far from pale. An excellent deep
sample contained 12 per cent, of cobalt oxide.
When properly prepared, cobalt green is a pigment of
EMERALD GREEN 219
great beauty and power.* The deeper tones of cobalt
green are almost transparent in oil. The pigment works
well, is quite permanent, and has no action on other pig-
ments. Cobalt green is, in fact, one of the too-rare pigments
which is at once chemically and artistically perfect ; such
at least is the conclusion I reached from my own trials,
but Mr. J. Scott Taylor tells me that cobalt green fades a
good deal when exposed to damp, although it stands light
well. It must be admitted, however, that it may be
exactly imitated by a mixture of viridian and artificial
ultramarine with a little zinc white.
Cobalt green is again coming into artistic use, as it is
equally adapted for all the methods of painting. It \vas
discredited for awhile by the inferiority of the product
obtained by Rinmann's original process (No. i above). It
ought not to be an expensive pigment.
Sometimes cobalt green is prepared by precipitating a
cobalt salt with an alkaUne arseniate or phosphate, and
then heating the precipitate with zinc white.
Emerald Green : Cupvic Aceto-Arsenite — Schweinfuvt
Green — Vert Paul Veronese — Schweinfurter Gri'in.
This pigment was discovered in 1814 during the course
of experiments made wdth the object of preparing an im-
proved Scheele's green. It may be prepared by half a
dozen slightly differing processes, but in all verdigris (or
vinegar and blue vitriol) and white arsenic are the two
essential materials employed. Generally verdigris is dis-
solved in acetic acid, and added to a boiling aqueous
* One sample of deep transparent cobalt green which I obtained
from a Paris colour-manufacturer contained both viridian and
ultramarine, added to enrich the colour of the cobalt green which
formed the basis of the pigment.
220 EMERALD GREEN
solution of white arsenic : on continued ebullition a de-
posit of emerald green occurs. Sometimes copper sul-
phate, potassmm arsenite, and acetic acid are employed.
Whatever the method, it is necessary that the coloured
product be washed with boiling water to remove the last
traces of soluble salts.
The hue of this pigment is a nearly normal green, slightly
verging upon bluish green ; it is brighter and more opaque
than Scheele's green, and, like it, is a deadly poison. It is
less attacked by sulphuretted hydrogen than Scheele's
green, but as a water-colour, does not long remain untar-
nished in impure air. In oil it is practically permanent,
both alone and when used with the majority of permanent
pigments. It is, however, quickly blackened by the cad-
mium yellows. Emerald green cannot be relied upon as
permanent in fresco and tempera painting. Its use in
wall papers and in the decoration of all domestic furniture
and fabrics is to be deprecated, by reason of its poisonous
character ; but is, happily, at the present time, in great
measure abandoned.
Emerald green, if pure, dissolves perfectly in boiling
dilute nitric or hydrochloric acid ; the solutions thus
made should yield no precipitate with a few drops of
barium chloride solution. An undissolved residue gener-
ally shows the presence of baryta white.
Emerald green may be distinguished from Scheele's
green by a simple experiment. If a small pinch of the
dry powder be warmed with a few drops of moderately
strong sulphuric acid (half oil of vitriol, half water), acid
vapours, having the smell of vinegar, will be given off only
when true emerald green is the subject of the experiment.
On the Continent, Vert Emeraude is the name given to
viridian, the emerald oxide of chromium.
SCHEELE'S GREEN 221
Scheele's Green : Cttpric Arsenite — Swedish Green —
Mitis Green — Scheeles Gn'in.
This pigment, discovered in 1778, is an arsenite of cop-
per with an excess of copper oxide. It is best prepared by
dissolving, in separate portions of hot water, white arsenic
and blue vitriol. The solutions are then mixed, and to the
mixture is added, in small successive portions, a solution
of potassium carbonate. These additions are stopped when
the precipitated pigment has attained its maximum of
colour intensity. In another process, a hot solution of
potassium arsenite is added to a hot solution of blue
vitriol. This pigment needs thorough washing with hot
water, and must be dried at a moderate temperature.
Scheele's green presents nearly the same characteristics
as emerald green, but is in every way inferior to that
pigment. It is eminently poisonous. It should not find
a place on the palette of the artist.
Vienna green, Mitis green, and Veronese green are
names which have been given to specially prepared
varieties of this cupric arsenite ; but there are very many
other designations by which pigments of essentially the
same composition are known. They are prepared by
slightly modified processes, and frequently contain such
foreign matters as chalk, heavy spar, or gypsum.
Malachite : Green Verditer — Green Bice — Mountain Green
— Green Carbonate of Copper — Vert de Montagne — Berg-
griin — Malachitgriin.
This green copper mineral was employed as a paint by
the ancients. It often accompanies the blue carbonate,
and occurs in many European, Asiatic, African, and
222 MALACHITE
American localities. The mines at Ekaterinburg and
Nischne Tagilsk in Russia, and at Burra-Burra in South
Australia, furnish malachite of fine colour ; it also occurs
abundantly in Namaqualand. Its variations in depth of
colour are due less to impurities than to differences in its
state of aggregation. Its specific gravity is about 4. Its
composition is that of a hydrate and carbonate of copper.
It may be represented by the formula CuCOgjCuHgOg. It
contains, therefore, less of the copper carbonate than
azurite, or blue verditer.
Malachite requires no other treatment than careful
grinding to fit it for use as an artists' pigment. The raw-
material must, however, be carefully selected, and all
visible impurities, such as ochreous veins and deposits,
and azurite, completely removed. An artificial malachite
was prepared and largely used in the seventeenth century,
and is still often substituted for the mineral ; but it is in-
ferior in colour and stabiHty to the native form.
Malachite as an oil-paint has often proved to be per-
manent, although it may seem to acquire a dull, brownish
hue, owing to the darkening and yellowing of the oil ;
sometimes, however, it becomes somewhat olive in
colour. In admixture with cadmium yellow it is liable to
blacken. It is so easily injured by impure air when un-
protected by any hydrofuge, that it is quite inadmissible
as a water-colour. In old tempera paintings it is some-
times found to have stood well ; but the sulphur from
the egg-medium and from the size has not infrequently
browned it.
Malachite is sometimes adulterated with baryta white ;
sometimes a mixture of that pigment and an arsenical
green is substituted for it. The former falsification may
be detected by boiling the sample in hydrochloric acid.
VERDIGRIS 223
when the malachite dissolves, leaving the baryta white
as a sediment. To detect an arsenical green, a small
portion of the sample should be mixed with powdered
charcoal, gently warmed at first in a long narrow test-
tube to drive off moisture, and then strongly heated ; a
dark sublimate of metalhc arsenic will form on the cooler
part of the tube.
Verdigris : Basic Copper Acetate — Vevt-de-Gris — Verdet
de Montpelliev — Gvilnspan.
This green copper pigment was called by the writers
of the fourteenth century 'viride Graecum,' or, more
simply, * viride,' ' viride terrestre ' being used for green-
earth — that is, 'terre verte.' * Vert-de-Grece ' — that is,
verdigris — was used by the ancient Romans as a pig-
ment, and has been detected in the wall-paintings of
Pompeii. It occurs in early Italian tempera pictures ;
but it has frequently injured the gesso-ground on which
it has been laid, forming calcium acetate with the calcium
carbonate, and disintegrating the surface. The blackness
of the shaded parts in many Venetian and Spanish pictures
of the sixteenth and seventeenth centuries has been attri-
buted to the changes which this pigment suffers in oil.
The medieval writers on the practice of painting en-
deavoured to show how the peculiar liability of verdigris
to change could be obviated by locking it up in some
hydrofuge substance, such as a resin or balsam. But the
problem actually possesses little practical interest to-day,
though of real moment in the study of old pictures. In
the modern palette the place of verdigris is taken by per-
manent greens derived from chromium and from cobalt :
concerning the safety of these we need not be anxious.
The large proportion of resinous matter employed by
224 VERDIGRIS
early painters for the protection of verdigris from alter-
ation, and the success of this precautionary measure,
may be seen in the green drapery in several pictures by
Van Eyck and Mabuse in the National Gallery and at
Hampton Court. It should, however, be stated that the
older processes for preparing verdigris often yielded a
product much more alterable in the presence of damp
than is the verdigris which for a century or more has
been made at Montpellier.
Verdigris is commonly called in chemical language a
basic acetate of copper. In fact, it is a mixture of three
such acetates, its varying hues, ranging from green to
greenish blue, being dependent upon the relative pro-
portions of these acetates. The most blue basic acetate
contains i molecule of copper acetate, and i of copper
hydrate, with 5 molecules of water ; the greenest has
twice as much acetate. Average verdigris contains in
100 parts about 29 parts of anhydrous acetic acid, 43 of
copper oxide, and 27 of water. It is nearly insoluble in
cold water ; but by continuous washing, or by continuous
exposure to moist air, is ultimately decomposed.
The Montpellier process for making verdigris consists
in exposing thin strips of metallic copper to the vapours
arising from grape marc undergoing the acetic fermenta-
tion. The operation is conducted in a moist, warm
atmosphere ; finally, the whole substance of the metallic
copper is transformed into verdigris.
An impure atmosphere containing sulphuretted hydro-
gen blackens verdigris; it is also affected by moisture
and by carbonic acid. As a water-colour, it is quite
inadmissible ; in oil, it stands pretty well if ' locked up '
in the way already described. But it acts energetically
upon several important pigments, and is very poisonous.
VERDIGRIS 225
For these reasons its employment in artistic painting
ought to be abandoned.
Verdigris, if pure, dissolves perfectly in liquor ammoniae,
any gypsum or barytes present as diluents or adulterants
remaining undissolved. If blue vitriol has been added to
verdigris, it also will dissolve in the ammonia ; but this
falsification may be detected by acidifying the ammoniacal
solution with hydrochloric acid, and then adding solution
of barium chloride — a white precipitate of barium sul-
phate indicates the presence of copper sulphate.
There are many composite green pigments sold by
artists' colourmen ; none is of real value. Green lake, a
mixture of quercitron lake and Prussian blue ; Hooker's
green — gamboge and Prussian blue ; olive green — Indian
yellow, umber and indigo ; and olive lake, a mixture of
quercitron lake, bone brown and ultramarine — all these
belong to the same category. However, there is one
mixed pigment, the so-called ' Cadmium-green ' on which
a favourable judgment may be passed : it consists of
viridian and cadmium yellow.
15
CHAPTER XVII
BLUE PIGMENTS
Ultramarine: Lapis Lazuli Blau — Lasuvstein Blau —
Outremer — Bleu d'Aziir.
There are at least three mineral species, closely allied in
chemical composition, and generally presenting a more or
less marked blue colour, which contain as their essential
constituents the five elements, silicon, aluminium, sodium,
sulphur, and oxygen, and which owe their characteristic
hue to the same compound. From one of these minerals, a
variety of ' hauyne,' often called ' lapis-lazuli,' the true or
native ultramarine is obtained. This stone occurs, of very
varying purity and colour, at Bucherei,Transbaikal,andin
many other Siberian localities ; at Ditro, in Transylvania ;
in the Andes of Ovalla, Rio Grande ; and in several regions
of Persia, Tibet, and China. It is the ' sapphire ' of ancient
authors. Small golden specks of iron-pyrites are frequently
irregularly scattered through its substance ; it is also very
frequently associated intimately with portions of the rocky
gangue, or matrix (limestone, syenite, granite, etc.), in
which it occurs. Very fine lapis-lazuli comes from Tibet.
To prepare a pigment from this mineral, selected pieces
of small size, as free as possible from pyrites or other im-
purities, are heated in a crucible and quenched (etonne)
in cold water, or very weak vinegar. The material, thus
226
ULTRAMARINE 227
disintegrated, is washed by decantation, and then dried
and carefully ground. The powder is then purified by
elutriation, or ' washing over,' the several wash-waters
depositing pigments of different depths of colour, and of
different degrees of fineness. Some manufacturers adopt
an old process, and make the powder into a soft mass with
a little rosin, linseed oil and beeswax, and knead, beat, or
macerate the lump, secured in a bag of coarse muslin
under very weak potash, or soda-lye — the alkaline water
carries off or withdraws the greater part of the pigment,
and deposits it on standing. The richness of the blue
product obtained depends primarily upon the original
quality of the stone, but several grades are always pro-
curable from the same raw material by means of the
above-described processes, bluish-grey and grey powders,
known respectively as ultramarine ash and mineral grey,
being the last and the least valuable products, while the
deepest and finest pigments are deposited from the
earliest wash-waters.
Optically, the superb blue of native ultramarine ap-
proaches more closely than the blue of any other pigment
to the pure normal blue of the solar spectrum ; it shows
very little violet and in this respect is unlike most speci-
mens of artificial ultramarine. Ultramarine is somewhat
harsh and granular in texture, a characteristic which may
be reduced by a small admixture of Chinese white, but
which becomes more marked when it is used as a light
wash, or in conjunction with transparent pigments, in
water-colour painting. It more nearly approaches trans-
parency when used in oil, and is then of excellent working
quality.
It is generally considered that ultramarine withstands
the action of light, moisture and sulphuretted hydrogen
228 ULTRAMARINE
perfectly, and that it neither affects nor is affected by any
other pigments. I have, however, been informed by an
English landscape-painter in oil, who has largely employed
native ultramarine in the skies of his pictures, that he has
lost faith in its inalterability. But the question arises,
' Was the pigment used always authentic ?' It dries well
in oil. It is decolourized at once by a hot solution of
alum, and ultimately even by a saturated cold solution,
which, however, bleaches very quickly all but the most
stable varieties of the artificial pigment. All mineral
acids, save carbonic, and all the common organic acids,
such as acetic, oxalic and citric, discharge the colour of
native ultramarine. It is only by a combination of
several tests in the hands of a skilled chemist that the
discrimination of genuine from spurious ultramarine may
be with certainty accomplished.
It may, however, be mentioned here that when a current
of hydrogen gas is passed over true ultram.arine heated in
a glass tube the powder retains its colour wholly or par-
tially for an hour or more, while the best artificial pig-
ment similarly treated becomes grey or greenish grey.
In the works — both in fresco and tempera, and in oil —
of many of the old masters, and in a large number of
illuminated manuscripts, the permanence of true ultra-
marine may be seen. If in some cases it has acquired a
greenish or dull hue in oil-painting, such change is due to
the yellowing of the oil and varnish, and not to any
deterioration of the pigment.
The price of genuine ultramarine is very high. This
is due less to the scarcity of the original lapis-lazuli from
which it is derived, than to the small yield and to the
elaborate and tedious operation by means of which the
pigment is prepared. But when every allowance is made
ARTIFICIAL ULTRAMARINE 229
on account of the troublesome and lengthy process of
manufacture, the cost of ultramarine is unwarrantably
excessive.
Artificial Ultramarine: New Blue — French Blue —
Permanent BUie — Gmelin's Blue — Guimefs Blue — Kiinst-
liches Ultramarin.
In the year 1814 a blue coloration, subsequently proved
to be due to ultramarine, was noticed in the soda (black-
ash) furnaces of St. Gobain. About fourteen years after-
wards a method of making the same blue substance at will
was discovered by Christian Gmelin, and by Guimet ; this
method was founded in part upon chemical analyses of
natural ultramarine, and in part upon a study of the con-
ditions under which the above-named blue coloration
occurred. By successive improvements in its manufac-
ture artificial ultramarine is now produced at a cost of no
more than a few pence per pound. It is chiefly made in
Germany and in France.
The raw materials employed in the preparation of ultra-
marine are kaolin, or China-clay, silica, sodium sulphate,
sodium carbonate, sulphur, charcoal and rosin. Some
makers omit the sodium sulphate, others the rosin, while
calcined alum is occasionally substituted for the kaolin.
These materials are heated together in closed crucibles in
a furnace, and slowly cooled. A greenish porous cake is
the product: this is powdered and gently roasted, after the
addition of a little sulphur, for some hours. The material
is again powdered, and then washed and dried : further
calcination is sometimes required to develop the proper
blue colour.
In preparing artificial ultramarine for use as an artists'
pigment it must be very finely ground, and very thoroughly
230 ARTIFICIAL ULTRAMARINE
washed with water free from lime. The grinding not only
improves the colour, but renders working with the paint
less difficult to manage ; the washing removes soluble
sulphates and certain sulphur-compounds, which are liable
to discolour some of the pigments (those containing lead
or copper) with which the ultramarine may afterwards be
associated in a picture.
The hue of artificial ultramarine is commonly of a less
pure quality than that of the natural pigment, verging
somewhat towards a purple. But its range of hue is
considerable, from a greenish-blue to a decided violet.
The greenish-blue and blue varieties are not affected in
hue by admixture with zinc white, but the varieties
which incline towards violet become remarkably enfeebled
in richness of colour by this admixture, such weakening
being out of all proportion with the dilution of tint which
would be expected to ensue from this addition of white.
However, other white substances do not produce this
curious result.
Weak acetic acid, and a saturated cold solution of
alum, which are without immediate action upon natural
ultramarine, generally change the hue, and always ulti-
mately decolourize the artificial product. Those kinds
which have a somewhat violet tinge resist the destructive
action of the above reagents longer than the pure blue
and greenish-blue varieties. Neither sulphuretted hydro-
gen, nor caustic lime or other alkaline substance, affects
the colour of artificial ultramarine.
Although the colour of ultramarine is certainly due to
a substance containing sulphur, the precise chemical
composition of this blue substance has not yet been
determined. Some chemists are of opinion that it is a
compound of aluminium, sodium, sulphur, and oxygen ;
ARTIFICIAL ULTRAMARINE 231
others regard it as a sulphide of aluminium. There is
little doubt, however, that it contains sulphur in two
conditions. Some curious derivatives of blue ultra-
marine, of various colours, have been obtained, in which
it is believed that the sodium of the original compound
has been replaced by other metals ; such is the yellow
'^silver-ultramarine,' prepared by keeping blue ultra-
marine in a solution of silver nitrate. These bodies are,
however, useless as pigments. The function of the silica
in ultramarine is not known, although it forms from
30 to 45 per cent, of the total weight of all the varieties,
and although it has been found by experience that ultra-
marines rich in silica resist the action of alum better
than those which are poor in this constituent : these
silicious ultramarines are sometimes sold under the name
of ' Oriental Blue.'
Artificial ultramarine, when properly prepared, is
permanent both in water and oils. When thin washes
on paper appear to lose strength as they dry, or soon
afterwards, the change is due to the chemical action of
the alum, or other aluminium compound, present in the
size of the paper. If an ultramarine should discolour
emerald green, chrome yellow, Naples yellow (true), or
flake white, it probably contains free sulphur, or has
been insufficiently washed.
In order to test the purity of ultramarine, an easy plan,
useful so far as it goes, is this : Boil a small quantity of
the sample with distilled water in a wide test-tube for
five minutes. Pour the liquid on to a wetted Swedish
filter-paper fitted in a funnel. Divide the clear filtrate
which runs through into 2 parts — to one add a few drops
of basic acetate of lead ; to the other a few drops of
barium chloride solution. No darkening should occur in
232 ARTIFICIAL ULTRAMARINE
the first case, nor any white cloudiness in the second.
Besides the white adulterants, gypsum and heavy spar,
both chessylite and Prussian blue have been found in
artificial ultramarine. If chessylite be present, its
presence may be detected by warming the sample with
ammonia solution, when a blue solution will be obtained.
Ultramarine containing Prussian blue acquires a brown
hue when warmed with caustic soda solution, while it
does not completely lose its blue colour when treated
with dilute hydrochloric or sulphuric acid.
When an acid (such as hydrochloric or oxalic) acts upon
ultramarine, it disengages both sulphur and sulphuretted
hydrogen ; a good deal of silica is also, in most cases, then
separated in a gelatinous form. The sulphur separated
as such may amount to as much as lo per cent, of the
weight of the pigment taken, or it may be less than a half
per cent. The sulphur disengaged as sulphuretted hydro-
gen through the action of an acid varies between a half
per cent, and 6 per cent. As these variations have no
relation to the depth of colour in the several samples, it is
evident that a great proportion of this sulphur does not
form an essential part of the blue pigment itself. Other
things being equal, it is well to select samples of ultramarine
which contain as little as possible of sulphur in any
state.
The ultramarines known as * Guimet's ' and ' Heu-
mann's ' are of fine quality. Large quantities of this
pigment are manufactured in Germanyand France, smaller
amounts in Belgium and England. The green, violet,
lilac, purple and red ultramarines of commerce are per-
manent pigments of some artistic value. They are or
may be produced in the manufacture of the blue variety,
the green being its precursor and the others being formed
ARTIFICIAL ULTRAMARINE 233
by further heating or treatment of the blue kind. The
final product is a greyish-white body.
There are two easily applied tests which, taken together,
enable one to estimate the relative values of a set of
samples of ultramarine in powder. These tests have dis-
tinct objects in view. In one, resistance to alteration is
determined — in the other, the colouring power. The
quantity operated on is only 5 centigrams, so the best
plan is to begin by weighing out, with the aid of a sensi-
tive balance, two portions of -05 gram a-piece from each
sample. One of each of these portions is thoroughly
mixed, by means of an ivory spatula, on highly glazed
white paper, with 2 grams of pure kaolin : the tints of
the several kinds are then compared. For the other test
we require a number of small precipitating glasses or
large test-tubes, and a supply of a saturated solution of
potash alum in distilled water. The five centigrams of
each sample of ultramarine are put into the duly labelled
glasses and a measured quantity (say 50 cubic centi-
metres) of the alum solution is poured on with constant
stirring. The change or loss of colour is duly noted at
intervals of time, some samples opposing a resistance of
days to the destructive influence of the alum, while others
are injured by a contact of a few hours, or, in some
extreme cases, of a few minutes. It is scarcely necessary
to add that the contents of each test-glass should be
thoroughly stirred at regular intervals. To avoid this
necessity, and at the same time to improve the accuracy
of the results, a slight modification of the method may be
introduced. Ten grams of the purest agar-agar are dis-
solved by the aid of heat in a litre of the alum solution.
Just before this liquid has become a jelly by cooling, a
measured portion is poured upon the necessary quantity
234 COBALT BLUE
of ultramarine : the mixture after thorough agitation is
allowed to set, so that the pigment remains suspended
throughout the mass ; any changes of colour can be
easily recognised.
Cobalt : Cobalt Blue — Bleu de Thenard — Kohalthlau.
Excluding smalt, which owes its colour to a cobalt
silicate, there are at least three pigments which go under
the name of ' cobalt ' or ' cobalt blue.' The best known
of these is a combination of alumina and cobalt oxide ;
then comes Leithner's or Thenard's blue, a cobalt phos-
phate on an aluminous base ; lastly, there is an aluminous
cobalt arseniate very much like the phosphate.
The original and simplest form of cobalt blue, or
Wenzel's blue, may be made by calcining strongly an
intimate mixture of aluminium hydrate and cobalt oxide.
A better way consists in moistening freshly-precipitated
aluminium hydrate with a solution of cobalt nitrate, dry-
ing and then strongly igniting the mass. It may likewise
be prepared by precipitating a solution of sodium alumi-
nate by means of cobalt chloride solution. Thenard's blue
may be prepared by mixing about 8 parts of aluminium
hydrate with i part of cobalt phosphate, both in the moist
condition, then drying and strongly calcining the mixture ;
cobalt arseniate may replace the phosphate. Another
variety of Thenard's blue is obtained by adding sodium
phosphate solution to a solution of alum containing a little
cobalt sulphate. In all the above methods, the complete
freedom from iron and nickel of the materials used is
essential to the purity and beauty of the blue pigment
formed.
The cobalt blues work well in all media. They are un-
affected by light, moisture, and oxygen. The best samples
CCERULEUM 235
of them are practically permanent even in impure air, but
ammonium sulphide tends to discolour them. If they
appear changed in hue in any oil-paintings, the yellowing
of the admixed or overlying oil or varnish must be regarded
as the cause. Cobalt blues may be used in fresco-painting ;
they are unaffected by commixture with other pigments.
Cobalt blues do not lose their colour when boiled with
alum solution, nor when treated with moderately strong
acids.
Cobalt blue as an oil-colour is usually ground with about
three-fourths its weight of linseed or poppy oil.
Cobalt blue examined optically is found to reflect much
green and violet Hght as well as blue. Viewed by candle
or gas light it acquires a very marked purplish hue.
Burton's cobalt, originally prepared for the use of potters,
is of fine quality and of more than usual stability.
The introduction of cobalt blue to the palette of the
artist may be said to have created a revolution in the
style of painting, especially obvious in water-colour land-
scapes.
CcERULEUM : Cenilium — Cerulean Blue — Coelinhlau —
Bleu Celeste.
When oxide of tin is moistened with cobalt nitrate solu-
tion and strongly heated, a greenish-blue mass is obtained,
which, after powdering and washing, constitutes one of
the varieties of the pigment known as coeruleum. There
are other ways of preparing this substance. One of these
consists in precipitating potassium stannate with cobalt
chloride, collecting and washing the precipitate, and then
mixing it with some pure silica and heating it. A good
specimen of cceruleum contained in 100 parts : 49-7 tin
binoxide, 18*6 cobalt oxide, and 317 silica. Some samples
236 PRUSSIAN BLUE
contain calcium sulphate, or lead sulphate, in place of the
silica ; they are of inferior quality.
Coeruleum is a permanent pigment of a rather greenish-
blue colour, without any tendency to the violet cast, so
noticeable with other cobalt blues (page 235), when viewed
by gas or candle light. It suffers little, if any, change by
exposure to light or impure air, or by commixture with
other pigments. It is a sub-opaque, rather earthy pig-
ment, with a moderate tingeing power. Although some
painters find it useful, coeruleum may be imitated so
nearly by a mixture of ultramarine, viridian, and white
that its presence on the palette can easily be dispensed
with.
Prussian Blue : Timibuirs Blue — Antwerp Blue — Berlin
Blue — Pnissiate of Iron — Chinese Blue — Saxon Blue —
Bleu de Berlin — Pariser-blau.
Although the chemical constitution of this pigment can
hardly be said to have been absolutely ascertained, yet it
is generally believed that there are at least three different
though closely allied chemical compounds included under
the above names, not to mention those varieties of this
pigment which contain added or extraneous substances,
such as alumina, plaster-of- Paris, or zinc-white. The
three typical and distinct compounds are :
I. Soluble Prussian Blue. — This is made by pouring a
solution of ferric chloride or ferric nitrate into an excess
of potassium ferrocyanide solution (yellow prussiate of
potash), or by pouring ferrous sulphate solution into
excess of potassium ferricyanide solution. The blue
precipitate formed is washed with distilled water until
the wash-water begins to acquire a blue tint. The com-
position of the pigment thus prepared is, when dry,
PRUSSIAN BLUE 237
represented by the formula K2Fe2(CN)j2Fe2. It con-
tains potassium, and is, in reality, a double ferrocyanide
— a ' potassio-ferric ferrocyanide.' It is less stable than
either of the other kinds of Prussian blue, while its solu-
bility in water causes it to stain the paper on which it is
spread in water-colour painting. It should invariably be
rejected by artists, although it must be owned that it
works very smoothly both in water and in oil. It may
always be distinguished from the superior kinds of
Prussian blue by very simple tests. One of these con-
sists in roasting a small portion of the dry powdered pig-
ment in a porcelain basin or iron tray, allowing the
brown residue to cool, and then throwing it into a little
pure water. Then place the mixture on a wetted filter
contained in a funnel, and see whether the clear filtrate
is alkaline by dipping a piece of yellow turmeric paper
into it ; if the yellow tint of this paper is reddened, then
the Prussian blue belongs to this section. Another test
is applied by simply washing some of the powdered blue
with warm distilled water on a Swedish filter — the filtrate
becomes blue.
II. Insoluble Pntssian Blue may be prepared by boiling
No. I. (the soluble kind) with a solution of ferric chloride,
by mixing solutions of ferrocyanic acid and ferric chloride,
by pouring potassium ferrocyanide solution into an excess
of a solution of ferric chloride, or of ferric nitrate, and
heating the mixture for some time, or by precipitating a
watery solution of Blue No. I. with an excess of either
of the above-named iron salts. It may also be obtained
by oxidizing Turnbull's blue (No. III.) with chlorine
water or nitric acid. The chemical composition of this
pigment is very complex, the simplest empirical formula
for it being Fe7(CN)ig: it will be seen that it contains no
238 TURN BULL'S BLUE
potassium. It always contains some combined water,
which cannot be driven off by heat without decomposition
of the salt. This blue is more permanent than No. I.
III. Tuvnhuirs Blue. — The chief constituent of the
original Turnbull's blue (more properly, Gmelin's blue)
closely resembles ordinary soluble Prussian blue, and,
like it, contains potassium. But the potassium may be
removed from it by stannous chloride solution, a sub-
stance being produced having the empirical formula
Fe5(CN)i2, but containing some water. Or the same
body may be made by precipitating a solution of ferri-
cyanic acid with a solution of ferrous sulphate or ferrous
chloride. This blue is of good colour, but is more diffi-
cult to obtain pure than No. II., the other insoluble
Prussian blue. Exposed to light, all the forms of Turn-
bull's blue, pure and impure, have a more decided ten-
dency to become greenish or to fade than No. II.
The ordinary commercial Prussian blue is a mixture,
in varying proportions, of the three blues above de-
scribed. It is made by adding green vitriol (ferrous
sulphate) solution to a solution of yellow prussiate of
potash (potassium ferrocyanide). The precipitate formed
(which varies in colour from a light to a deep blue,
according to the amount of ferric salt present in the
green vitriol) is then oxidized by means of dilute nitric
acid or of a solution of bleaching powder. After having
been washed, the substance is treated with hydrochloric
acid, and is then again washed with water.
All the above blues are of a very deep blue colour
in powder or in the lump, but when pressed or rubbed
they all show a coppery lustre. The only one fit for
artists' use is the insoluble variety (No. II.), the others
being less stable or having other defects. The insoluble
PRUSSIAN BLUE 239
form is, moreover, the only one which yields, when
roasted, a perfectly satisfactory ' Prussian brown.'
Prussian blue is a transparent colour of great force and
richness, and works well in oil as well as in water. In
thin washes or layers it has a slightly greenish hue. Its
colour is changed by lime and by the weakest alkalies, so
that it cannot be employed in fresco or on newly-plastered
walls. Long-continued exposure to strong light weakens
and alters the colour of Prussian blue, but the insoluble
varieties are less affected than the soluble. When this
fading of the pigment in water-colour washes has taken
place, a brief sojourn in darkness generally suffices to
restore the hue almost to its original depth and quality.
This strange phenomenon, which awaits explanation, has
been long familiar to artists' colourmen. The influence
of moisture in determining the fading of Prussian blue
under solar exposure is seen in three comparative trials
with water-colour washes on paper. In a sealed tube
with ordinary air the intensity was reduced from 10° to
1°, and the colour became sea-green in thirteen months ;
after four years a part of the same wash retained its full
depth when the slip was exposed in air kept dry ; another
portion was reduced to 8-5° by four years' exposure in
an ordinary frame. A sample of Prussian blue (as
ordinarily made) in oil, after five years' exposure, had
become somewhat greenish, with a loss of about one-
tenth of its depth. These changes were more obvious in
the pure transparent pigment than in its tint with flake-
white. A second specimen, from another maker, similarly
exposed, was rather less affected, both as to loss of colour
and as to change of hue.
Prussian blue was discovered in 1704 by a colour-
maker of Berlin, Diesbach by name. When, as is some-
240 CYAN IN E
times the case, this pigment is found in water-colour
paintings of the seventeenth century, it is scarcely neces-
sary to state that its presence betrays the brush of the
restorer or the forger.
Antwerp blue is a sort of Prussian blue lake, the pig-
ment consisting of a colourless base dyed with Prussian
blue. According to one method of preparation, a solution
of I part of green vitriol and 2 parts of zinc sulphate in
40 parts of water is precipitated by adding to it a solution
of 4 parts of potassium ferrocyanide in 40 parts of water.
The blue colour of the precipitate deepens as it is washed.
Alumina is sometimes used as the base upon which the
blue pigment is thrown. Antwerp blue is less transparent
and less intense in colour than Prussian blue ; it has
about the same degree of stability.
Cyanine : Leitch's Blue.
A mixture of Prussian blue and cobalt blue has been
sold under the name of cyanine. It would seem from
some recent experiments made with this mixed pigment
that it is fairly permanent, even in water-colour painting.
It is, of course, not adapted for use in fresco, as the Prus-
sian blue in it at once yields rust through the action of the
lime of the intonaco. Mixed pigments cannot, however,
be recommended, as it is in nearly all cases better for the
artist himself to associate together those paints which he
wishes to mingle. This plan gives him the opportunity
of ascertaining the purity and quality of the several com-
ponents of his mixtures. In the case of cyanine, it
appears that the more permanent constituent, the cobalt,
partially protects the Prussian blue from change — a result
which is still better seen v/hen a separate wash of cobalt
is laid over a wash of Prussian blue. In this case the
INDIGO 241
light which has penetrated through the cobalt particles
would seem to have been deprived of those rays which
effect the decomposition of the Prussian blue beneath.
Such a phenomenon is not unusual, and has been ob-
served in the case of several pairs of pigments having
very closely-allied hues.
Indigo.
Indigo has been used either as a pigment or a dye from
very early times in India and in Egypt. It is referred to
under the name of indicum by Pliny; later on the Byzantine
writers called it azovmni Romanum. ' Indigo bagadel ' —
that is, indigo of Bagdad — is named as early as 1228 in
the Marseilles tariffs ; in the early English accounts rela-
ting to painting works (1274) it is called ' indebas.' In
the fourteenth century it was designated as ' ind,' ' inde,'
and ' ynde.* ' Endego ' and ' indico ' were used in the
sixteenth and seventeenth centuries. It was first largely
imported from India into Europe in the seventeenth cen-
tury by the Dutch.
A large number of different plants yield true indigo.
This pigment was once obtained in considerable quantity
from a crucifer, I sat is tinctoria, the dyers' weed or woad,
the ' pastel ' of the French ; but the chief source is now
Indigofera tinctovia, a leguminous shrub, probably of
Indian, or at least Asiatic, origin.
Indigo (CjgHjoNgOg) does not exist ready-formed in the
plants which yield it, but occurs in the form of a colourless
compound, or glucoside, which, by combining with water,
splits up into a sugar and indigo. It is prepared from the
freshly-cut plants, or from the dried foliage, by maceration
in water and fermentation, followed by boiling (sometimes
lime-water is first added) ; the dark precipitate which forms
16
242
INDIGO
is thrown on to cloth-strainers, and finally pressed and
dried. The mineral impurities which commercial indigo
contains are derived partly from the plant itself, partly
from the water used in preparing it, and partly from the
lime-water above mentioned ; moreover, it is sometimes
adulterated. Indigo is easily oxidized by a very large
number of substances rich in oxygen, yielding a yellow
product called isatine ; it is converted into a colourless
body (CjgHjgNgOg) by many reducing agents.
Indeed, several of the processes of purifying indigo
depend upon the reduction of the blue colouring sub-
stance, or ' indigotin,' into ' white indigo,' and the subse-
quent precipitation of the blue matter by exposure to the
oxygen of the air. Green vitriol is the commonest reducing
agent, and is used in association with lime. The purified
indigo prepared by this process, though of fair colour, does
not, however, work so well as a paint as the best Bengal
indigo treated successively with acid, alkali, acid, and
alcohol. Indigotin, if quite pure, has a somewhat purplish
cast in thick water-colour washes ; this hue is observable
with this substance whether obtained by sublimation or
by Fritzche's process with grape-sugar, caustic soda, and
alcohol.
The impurities in commercial indigo constitute from
20 up to 70 or 80 per cent, of its total weight — the
average is about 50. They consist mainly of mineral
matter, indigo red, indigo brown, and nitrogenous com-
pounds. Much of the mineral matter may be removed
by digestion in hydrochloric acid, followed by treatment
with boiling water. Sodium hydrate solution dissolves
the indigo brown, while strong alcohol takes away the
indigo red, which amounts to nearly 4 parts in 100 of
the original indigo. After treatment with these three
INDIGO 243
solvents, the residual purified indigo is of an intense and
very beautiful hue. Java indigo is generally of very
good quality, that from Bengal comes next, and then
the indigo from Oudh, Kurpah, and Madras. Japanese
indigo is generally poor.
Indigo frequently receives no purifying treatment
previous to its being ground into a fine powder suitable
for admixture with oil or with gum and the other media
of water-colours. The necessity of choosing the purest
and finest samples of the commercial dye-stuff is of
course evident, but it is better in every case to adopt the
processes of purification named in the preceding para-
graph. No sample of purified indigo should leave, after
being burnt, more than 3 per cent, of ash.
This rich and transparent blue is, unfortunately,
gradually oxidized and browned when exposed to light.
In thin washes of water colour it disappears rapidly in
the sun's rays, much more slowly when submitted to
diffused daylight. The following figures approximately
represent the reduction in force of a sample of indigo as
a moist water-colour when exposed to sunlight :
Original intensity ... ... ... 10
After two years ... ... ... i
After ten years ... ... ... o
Other trials with other samples gave in some cases less
unfavourable results.
Indigo in cake is sometimes less affected by sunlight
than the moist preparations. As an oil-colour, indigo
loses from one-third to one-half of its intensity when
exposed to sunlight for five years, its hue being at the
same time altered, in different specimens, either to a
greyish or a greenish blue ; the change is more con-
244 INDIGO
spicuous when the indigo has been mixed in tint with
flake or other white. Locked up in copal or amber
varnish it is more slowly changed. The fading is due to
oxidation.
Indigo may be replaced advantageously by ultramarine
mixed with a trace of viridian, or by a good Prussian
blue, either being associated with a little ivory-black.
Several pigments, such as aureolin, true Naples yellow,
and all the chromates, have a very marked effect upon
indigo.
In order to ascertain whether the fading of indigo as a
water-colour, on exposure to sunshine, was increased by
the presence of alum in the paper, a series of comparative
experiments were made. A pale tint of indigo was
spread upon (i) paper free from alum; (2) paper washed
with alum solution ; (3) paper containing a trace of
alum ; (4) paper which had been washed with weak
ammonia-water after having received an alum-size.
After six months' (April to September inclusive) exposure
to sunlight, all the four specimens showed complete
extinction of the blue pigment, the disappearance of the
colour from No. 2 having, however, been a trifle more
rapid than in the other cases. Honey and glycerin,
owing to their hygroscopic character, appear to hasten
the fading, so does water-vapour; for a dried sHp of
indigo-washed paper sealed up in a glass tube loses its
colour less quickly than one in its ordinary moist con-
dition, when both are exposed side by side to sunshine.
A tightly-framed water-colour drawing presents, of course,
a close analogy to the second or more unfavourable set of
conditions. When a medium tint of indigo on paper was
exposed for four years to sunlight in a tube containing air
kept dry by a water-absorbing agent, its original depth of
INDIGO 245
colour was perfectly preserved. An identical experiment
in an ordinary glazed frame resulted in a reduction of
tone from 10° to 1°, while the residual hue was a greenish
grey.
The effect, if any, of the several iron reds, such as light
red, Venetian red, and Indian red, in accelerating the
fading of indigo in sunshine has been tested in many ways.
Provided the three red pigments above-named be free
from soluble salts, and especially from sulphates, they are
equally innocuous ; the indigo disappears with or without
them at the same rate. In one instance only a wash of
Venetian red with indigo lost its indigo completely three
weeks before the destruction of the blue in the parallel
experiments was complete, which was the case in six
months. It is impossible to assert that this result was
not due to a smaller amount of indigo having been pres-
ent in this particular case, but I believe it to have been
due to this sample of Venetian red having been an imita-
tive one prepared from colcothar.
In comparing under the same conditions the relative
stability, when exposed to light, of different indigoes, it
appears that ' Bengal refined ' is superior to the best ' raw
Bengal,' and even to the indigotins obtained by the green
vitriol and glucose processes.
The superiority, when exposed to light, of refined indigo
— that is, of indigo which has been purified by the treat-
ment with acid and the other solvents already named —
although observed in recent experiments, seems to have
been ascertained in the time of De Mayerne. One of the
authorities (Elias Feltz, of Constance) quoted by De
Mayerne affirms that this pigment may be rendered safe
by steeping it in vinegar and exposing it to the sun for
several days ; the vinegar is then to be poured off, and
246 INDIGO
the paste when dry ground in oil. Another plan recom-
mended (in a note, dated 1642) by Feltz consists in grind-
ing the indigo with a little calcined alum ; this plan is,
however, objectionable for several reasons. The treatment
of the indigo with the solvents previously named, which
are adapted to remove its natural impurities, is the only
legitimate plan. It is scarcely necessary to add that there
is abundant evidence to prove that in De Mayerne's day
indigo was generally regarded as an unstable colour.
Under all circumstances no indigo should be employed in
painting unless it be completely free from acids.
Although it is not possible to ascertain the exact rich-
ness in indigotin of commercial indigoes by any tests save
those in use by professional chemists, yet it is easy to
learn a good deal about any particular sample by means
of a few simple experiments. A good sample should
appear homogeneous, and should float on water. Dried
at 100° C, it should not lose more than 6 parts per 100 of
moisture. When rubbed with a hard smooth substance
it should show a coppery lustre. One hundred grains
when burnt should not leave more than 10 grains of grey
ash. It should dissolve perfectly in four times its weight
of fuming sulphuric acid. Starch, gypsum, clay, chalk,
steatite, and Prussian blue are amongst the adulterants of
indigo.
Indigo dissolved in four times its weight of fuming oil
of vitriol (kept cool) forms a liquid from which, after
slight dilution and filtration through asbestos cloth, potas-
sium carbonate precipitates a fine blue compound, which
has been used in water-colour painting under the name of
indigo carmine. It is not a safe pigment.
The artificial or synthetic indigo, although characterized
by freedom from impurities, both organic and inorganic.
INDIGO 247
does not offer any advantages over the natural product,
if properly purified, when employed as a paint in water-
colour or oil. It may be added that those derivatives of
indigotin, in which some of the hydrogen atoms have been
substituted by bromine or chlorine, are no more perma-
nent than the original dyestuff. This is the case with
that form of dibrom-indigotin (the punicin of the late
H. E. Schunck), which occurs naturally in, or rather was
obtained from, certain marine molluscs [Purpura lapillus
and Murex brandaris), and which has been known for
many centuries as Tyrian Purple. The very minute yield
of the natural pigment would have made its general use
impracticable, but now that the identical substance can
be freely made from indigo, it is found that its lack of
permanence should exclude it from the 'selected' palette
of the artist of to-day.
In examining old water-colour drawings, it will often be
found that the parts protected by the mount or frame show
the indigo used in compounding the greys of clouds and
the greens of vegetation perfectly intact, while it has com-
pletely left the exposed parts. In many cases where indigo
is supposed to have withstood long exposure to light, it
will be found that the blue used has been Prussian blue,
modified by admixture with other pigments. But in many
old pictures and drawings it will be found that the un-
natural bluish hue of the foliage represented is due to the
complete loss of rapidly evanescent yellows, rather than to
the entire stability of the indigo with which the greens have
been compounded. Nor must it be forgotten that indigo
was not the only blue in such mixed greens, blue carbonate
of copper and even lapis-lazuli having been extensively
employed at that time. The latter pigment is perfectly
permanent except in the presence of acids and alum.
248 SMALT
Smalt : Royal Blue — Dumonfs Blue — Zaffre — Bleu de
Smalte — Smalte — Zaffev.
Glass and vitreous mixtures containing cobalt, and of
a rich blue colour, have been known for ages, although
copper, not cobalt, was the colouring principle of most of
the antique materials of this class. The smalt now made
is a very deep-blue glass (appearing black in the lump),
consisting essentially of cobalt and potassium silicate. It
generally contains, in 100 parts, silica, 65 to 71 ; potash,
16 to 21 ; and cobalt oxide, 6 to 7 parts : a little alumina
is always present. In the inferior varieties the oxides of
iron and nickel always occur.
In order to prepare a pigment from the dark cobalt glass,
it is fused, and then poured into cold water. After the
disintegration thus effected, the glass is ground into a
moderately fine powder, and submitted to the process
called elutriation, or washing-over. The finest particles,
which take longest to settle from the wash-waters, are
the palest in colour ; the larger particles, though of richer
hue, are very difficult to use as a paint.
Smalt is rarely employed now as an artists' pigment,
cobalt blue and artificial ultramarine having been very
advantageously substituted for it. For not only is it a
difficult pigment with which to work, in both water and
oil, but it is gradually altered by moisture and by the
carbonic acid of the air, becoming paler and greyer ; more-
over, the finer the state of division in which it exists, the
more rapid is the change. It will be found that even
spring or distilled water is competent to start the decom-
position of smalt. For if a little of this pigment be placed
on a piece of yellow turmeric-paper, and moistened with
clean water, there will soon be formed a red stain beneath
CHESSYLITE 249
the smalt — proof of the Hberation of some of the alkaUne
constituent (potash) of this blue glass.
Chessylite : Bhce Vevditev — Bice — Mountain Blue —
A ziivite — Cendves Bleues — Bevghlau.
This copper mineral differs from malachite in containing
less hydrate, or more carbonate of the metal, its composi-
tion being represented by the formula aCuCOg, CuHgOg.
Its best-known locality is Chessy, near Lyon, in France,
but very fine specimens are found at Wallaroo and
Burra-Burra, in South Australia, and in the district of
Perm, Siberia.
This blue pigment has been prepared artificially, but
the natural substance is far less liable to change on
exposure to impure air. The introduction of cobalt blue,
and more particularly of artificial ultramarine, has
practically caused the disuse of blue verditer, or rather
of chessylite ( = azurite), this beautiful native blue pig-
ment, which, when used in illuminated manuscripts, as
was largely the case between the thirteenth and six-
teenth centuries, has kept its hue unimpaired to the
present day.
Under the name of Bleu Lumi^re a beautiful turquoise-
coloured paint was introduced to artists. It consists
mainly of an artificial copper hydrate along with some car-
bonate. It becomes greenish by exposure to sunlight or
a very moderate degree of heat. Like all similar artificial
compounds of copper it proves to be untrustworthy.
Many old pictures, especially those in tempera, afford
evidence of the blue hydrato-carbonates of copper having
become green. With the extensive range of more per-
manent blues at the command of the modern artist,
there is no need to retain this pigment on the palette.
250 BLUE PIGMENTS
Egyptian Blue.
A beautiful and permanent blue pigment, generally
known as Egyptian blue, has been studied by many
chemists. It is found on objects of Egyptian origin
from the time of the fourth Dynasty onwards, and
was in use during the time of the Roman Empire.
Specimens have been found in Pompeii and other Roman
sites ; also in Britain, at Wroxeter, in Syria, and in
Crete. Our knowledge of this pigment has become more
exact in consequence of the researches* of Professor A.
P. Laurie, who has re-determined its chemical composi-
tion as well as its physical properties, and has, moreover,
ascertained the precise conditions under which it is pro-
duced. In one of Professor Laurie's trials he took i8o
parts of fine sand, 48 copper carbonate, 36 calcium car-
bonate, and 20 parts of ' fusion mixture.' A few grams
were submitted to definite temperatures in an electric
furnace for some hours, the mass being cooled, re-ground,
and re-heated. Finally it was found that the optimum
temperature for the production of the crystalline blue
was somewhere about 830° — 850° C. The crystals,
which are transparent and dichroic, and have a density of
3, consist of a double silicate of calcium and copper,
represented by the formula CuO,CaO,4Si02, but gener-
ally containing about 2 per cent, of potash and soda, as
substituents for a part of the copper and calcium oxides ;
the actual percentage of copper oxide present in the
pure blue is therefore usually somewhat lower than the
theoretical. It should be added that the interest at-
tached to the subject of Egyptian blue is rather academic
* Proceedings of the Royal Society, A. (1914), vol. 89, pp. 418-429.
MANGANESE VIOLET 251
than practical, for this pigment is not now made except
on a small laboratory scale, and has not found for many-
centuries a place on the palette of the artist.
Manganese Violet : Mineral Violet — Permanent Violet —
N limber gevviolett .
This pigment is rarely met with on the palette of the
artist ; it is, however, quite permanent and has a truer
violet hue than cobalt violet, which is redder as well as
brighter. Its preparation is somewhat tedious. It is
made by means of manganous chloride and phosphoric
acid, solutions of these compounds being mixed together,
evaporated to dryness, and then the residual mass fused.
The fused mass is broken up and boiled with a solution
of ammonium carbonate. The turbid liquor is allowed
to settle, and then the clear portion is decanted or filtered
off and evaporated to dryness, and the residue fused. By
grinding the fused mass and boiling it with water a fine
precipitate of ' manganese violet ' separates : it is re-
moved by filtration and thoroughly washed and dried.
It appears to be essentially manganous metaphosphate,
although by no means pure.
Two other violet or purple pigments of a permanent
character have been already described. One of these is
violet cobalt (page 211), the other violet ultramarine
(page 232).
CHAPTER XVIII
BROWN PIGMENTS
Raw Umber : Levant Umber — Terre d'Ombre — Umbraun —
Umbra — Terra Ombra.
This earth is found in several localities ; the best variety
has come for some time past from Cyprus. A consider-
able number of Cypriote specimens, of several nuances,
some excellent, were shown in the Colonial and Indian
Exhibition of 1886. It differs chemically from the yellow
and brown ochres in several particulars, notably in the
presence of a considerable quantity of one of the higher
oxides of manganese (Mn^O^, or MnOg), and in the small
proportion of water which it contains. Samples from
English localities are poor in iron ; one Derbyshire
specimen gave Mr. G. H. Hurst no less than 30 per
cent, of barium sulphate. An analysis of my own, made
with a choice sample from Cyprus, showed the following
percentages :
Water, lost at 100° C. - 4*8
Water, lost at a red heat*- 8*8
Iron oxide (FcgOa) ' - 48 '5
Manganese dioxide (Mn02) i9"o
Lime - - - - - i"4
Magnesia - - - - 0*5
Alumina - - - - 2-1
Phosphorus pentoxide - 0*9
Silica 13-7
Carbon dioxide, etc. - - 0*3
This sample had the peculiar greenish hue so much
* Includes a little organic or bituminous matter.
252
RAW UMBER 253
prized by artists. It should be stated that a part of the
manganese probably existed as MngO^.
Before being used as a paint, this brown mineral is
finely ground, washed with water, and then dried at
100° C, or at a slightly higher temperature. When, by
a stronger heat, the whole of its water has been expelled,
the umber acquires a reddish hue, and is then the pig-
ment known as burnt umber. This change of colour is
due to the passage of the brown ferric hydrate into the
red ferric oxide, and to an increase in the proportion of
the red-brown manganese oxide present.
Raw umber in powder, after having been purified, soon
acquires a very slight reddish hue on exposure to light
and air ; it is a good plan to place the undried, finely-
ground mineral on trays in the sunshine before completing
its desiccation and mixing it with oil or other medium.
Raw umber is permanent when used with each or any
of the painting media : the slight yellowish or dull aspect
which it acquires in oil may be traced to the augmented
translucency of the paint, and to the yellowing of the
associated oil. Umber is without action on other pig-
ments. A very few samples of umber, used as a water-
colour, have been observed to fade slightly after from
five to ten years' exposure to sunlight. But this
deterioration is due to the presence of traces of brown
peaty acids, or ' humus ' substances, which occasionally
occur in the umbers from certain localities.
Raw umber possesses a semi-opaque, citrine-brown
colour ; it works and dries well in oil. Associated with
transparent blues, it yields soft, quiet green hues ; it is
invaluable both in figure and landscape painting. As a
priming or first painting-ground, it is apt, like most dark
pigments, to become more conspicuous in time, owing
254 RA W SIENNA
chiefly to the translucency which the superimposed
painting gradually acquires.
Raw umber is not subject to aduheration, but a ferru-
ginous brown coal has been occasionally substituted for
the true mineral. The great variation in quality shown
by the umbers of commerce is due, in great part, to the
difficulty of securing, even from the same mine, con-
tinuous supplies of the same excellence.
Burnt Umber.
It has before been pointed out that raw umber, from
which burnt umber is prepared by calcination, is not an
ordinary ochre, but owes its colour in great measure to
the presence of a considerable amount of some compound
of manganese. The exact constitution of raw umber is
not, however, known, although the slight change in hue
which occurs when it is roasted negatives the idea that
it contains any considerable proportion of the ordinary
ferric hydrates. If these were present in notable propor-
tions, roasting would certainly redden raw umber much
more than is actually the case.
Burnt umber differs in quality and hue from the raw
earth mainly in being more translucent, and of a warmer
brown. Although some Continental authorities affirm
that this pigment darkens and becomes purplish in course
of time, I cannot regard it as otherwise than perfectly
permanent, and as exerting no action on stable pigments.
Raw Sienna : Terre de Sienne — Terra di Siena — Rohe
Sienna.
This earth, which is found chiefly in Tuscany and the
Hartz, is a particularly rich variety of yellow ochre, and
contains a large proportion of a ferric hydrate. The late
RA W SIENNA 255
G. H. Hurst (' Chem. News,' vol. lix., p. 172) gave three
analyses of raw sienna. In these the range in the
percentages of the more important constituents is as
follows :
Hygroscopic water ... ... 8-2 to 17-5
Combined water "^ ... ... 9*0 „ i2'4
Manganese dioxide ... ... o-6 „ 1-5
Iron oxide (FegOg) ... ... 45"8 „ 597
Silica 5'o „ 17*4
Raw sienna is prepared for use by crushing, sifting to
remove sand, grinding and washing in the same way as
in the case of yellow ochre. It should be noted that
when lumps of fresh raw sienna are first broken and
exposed to the air, their surfaces acquire a slight olive-
green nuance. In order to avoid any disadvantage which
might arise from such an alteration taking place subse-
quently, it is a good plan to expose the crushed earth to
the air and light previous to its final preparation as a
pigment. Before grinding it in oil, it should be cautiously
dried at 60° C. to remove the greater part of the accidental
or hygroscopic moisture.
Raw sienna in thick washes is somewhat deeper in
tint and of a warmer and browner hue than yellow ochre.
Owing to a trace of organic or peaty matter which it
contains, raw sienna is liable to become rather less brown
and more yellow by long-continued exposure to strong
light. It is generally without action on other pigments,
and is available for water-colour, oil, and tempera paint-
ing ; it sometimes fails in fresco. A given weight of raw
sienna requires a larger proportion of oil than any other
pigment ; the finished oil-paint contains only 30 per cent,
of pigment.
* Includes traces of organic matter.
256 CALEDONIAN BROWN
Raw sienna is not subject to adulteration, but it is well
to be certain that the pigment has been well washed.
Burnt Sienna.
The roasting or calcination of raw sienna produces a
very great change in its hue as well as in the depth of its
colour. The ferric hydrate of the raw earth becomes
wholly converted into ferric oxide, this change being
accompanied by a great increase in the translucency and
depth of the colour.
When small fragments (or the coarse powder) of raw
sienna are calcined, it will be noticed that the change of
hue which occurs is not quite uniform, some points being
of a brighter and redder brown than others. A very fine
powder, thoroughly stirred during roasting, shows this
peculiarity in a much less marked degree.
Burnt sienna possesses a very beautiful, warm, reddish-
brown hue, which cannot be exactly imitated in trans-
lucency and depth by mixtures of other pigments. It is
permanent, without action on other pigments, and not
liable to adulteration. It is available for use in every
method of painting. Some fine and permanent foliage-
greens may be made by associating viridian with burnt
sienna.
Caledonian Brown.
This brown, although a natural earth, presents very
much the appearance of burnt sienna. It contains a
small quantity of combined moisture. It consists mainly
of the brown hydrates and oxides of manganese and iron.
When calcined it loses its ruddy hue and becomes almost
black — a black with a slight brownish hue.
Caledonian brown, whether raw or burnt, is a per-
VANDYKE BROWN 257
manent and innocuous pigment, which is well adapted
for oil and tempera painting. It is said that the original
source of this pigment is exhausted, and that an imita-
tive mixture of burnt sienna and bituminous Vandyke
brown is sold in lieu of Caledonian brown.
Vandyke Brown.
Three brown pigments pass in commerce under the
name of Vandyke brown. The first is made by calcining
certain very ferruginous earths or brown ochres ; the
second is nothing more than a dark-brown variety of
colcothar ; the third is a kind of brown earth containing,
along with some iron oxide and hydrate, a good deal of
organic substance in the form of humus or bituminous
matter. The first and second kinds are permanent and
innocuous, but the third kind will not resist the pro-
longed action of light, becoming paler and redder in the
course of time. The discrepancies in the published state-
ments as to the permanence of this pigment are thus
readily explained ; it is to be regretted that most of the
samples of Vandyke brown now met with in England
belong to the third kind, and therefore fade quickly in
water-colour, more slowly in oil. This sort may be
recognised by the dark sublimate which it yields when
its powder is heated in a test-tube, as well as by the
change in colour and great loss of weight which it then
shows. We shall designate this less satisfactory variety
in the Tables of Permanent and Fugitive Pigments in
the present volume as Vandyke brown B., the other kinds
being called Vandyke brown A. It is unfortunate that
the colour-value of the perishable variety is incom-
parably greater than that of the more permanent sort.
17
258 CAPPAGH BROWN
Cologne or Cullen earth, and Cassel brown or Cassel
earth, are soft, impure varieties of brown coal or lignite.
They vary in fixity, some of them being even more easily
bleached by light than Vandyke brown B. ; these should
not find a place on the palette of the artist. When
slightly roasted, a part of the brown organic matter in
these earths is charred or carbonized, and the substance
becomes darker, duller, and decidedly less alterable by
exposure. Some of the so-called Cologne earth now sold
is merely Vandyke brown B., slightly changed by gentle
roasting ; it is then rather less alterable. We have met
with some specimens of Cassel earth which proved prac-
tically permanent in oil, but even these faded quite
distinctly when exposed to strong light after having been
mixed with flake-white.
Cappagh Brown : Euchrome — Mineral Brown.
This earth was found on the estate of Lord Audley, in the
Cappagh Mine, which was opened in the year 1814, and is
situated about ten miles west of the town of Skibbereen, in
the county of Cork. In composition and general characters
it resembles raw umber, but has a more reddish hue. It
contains ferric hydrate and ferric oxide, with a consider-
able amount of one of the oxides or hydrates of manganese.
It gives off a good deal of water when heated to 100° C,
and acquires a rich reddish-brown colour, not unlike that
of burnt sienna, and almost identical with that of Cale-
donian brown. The specimens examined contained mere
traces of organic matter, so that it cannot be regarded as
a kind of bog-earth or peat, although it is, of course,
possible that there may be another mineral found in the
same locality which might be correctly so designated. The
BISTRE 259
following analysis represents the composition in 100 parts
of a characteristic specimen of this pigment :
Water, lost at 100° C. - 187 ^ Alumina - - - - 2-6
Water, lost at a red heat - ii-6 | Lime - - - - - 11
Iron oxide (Fe203) - - 34'4 j Silica - - - - - 4*6
Manganese dioxide (MnOg) 27-2 | Phosphorus pentoxide(P205) 0-4
It is probable that a part of the manganese really exists
in the form of the red oxide (MngOJ, and a part of the iron
as ferrous oxide (FeO). I have, however, calculated both
these metals into their higher oxides. In this way it
happens that the added percentages exceed 100, even when
the traces of magnesia and potash present in Cappagh
brown are not included in the total. The large quantity of
water present in this mineral in a loosely-attached form
(hygroscopic), amounting to nearly one-fifth of the weight
of the pigment, indicates the desirability of cautiously
drying the substance previous to grinding it in oil. A
temperature of 60° C. should not be exceeded.
Cappagh brown works well in oil, particularly if it be
dried at a heat below that of boiling water before it be
ground in oil. It is an innocuous pigment, but its perma-
nence has scarcely been sufficiently tested. A rub of
Cappagh brown in oil, exposed to strong sunlight for
one month, lost a little of its yellow, assuming a some-
what ruddier hue ; the change, however, was very slight,
and did not appear to have increased after continued
exposure for five months more.
Bistre : Bister — Bvaunev Lack — Russhvaim.
Bistre is prepared from the tarry soot of certain woods,
especially from that of beech-wood, by the following pro-
cess : The soot is finely ground and sifted, and then the
26o ASPHALTUM
powder is digested with successive portions of hot water
until the latter no longer acquires a brown or yellow tint ;
the residue is then ground with suitable quantities of gum-
water and glycerin, and preserved in the moist state. To
form cake-bistre the glycerin is omitted, but more gum is
employed. Bistre is not used as an oil-colour.
The tarry matter in bistre is its element of weakness.
By exposure to strong light this tarry matter oxidizes, and
the residual pigment becomes cooler in hue and paler.
Professor Ogden Rood, experimenting with a weak water-
colour wash of bistre, found that it lost 19 parts of its
original intensity of 25 by an exposure of three and a
half months to the summer sun. I have not found so
considerable a change to occur with the samples of bistre
which I have tried ; but this pigment varies much in
composition, being obtained from the soot of different
woods as well as from that of peat. The most fugitive
preparations are those made from samples of soot con-
taining the highest proportions of tarry matters.
AsPHALTUM : Bitumen — Mineral Pitch — Antwerp Brown —
Mummy — Mumie.
Asphalt, asphaltum, or mineral pitch, has long been
used as a pigment. The best known is that from the
Dead Sea {Laciis asphaltites). Other abundant sources of
this carbonaceous mineral occur in Trinidad ; Caxitambo
and Berengela, in Peru ; Val de Travers, Neufchatel ;
Avlona, in Albania, etc.
Asphalt is rather a mixture of minerals than a single
mineral ; it is, moreover, very variable in the nature,
character, and proportion of its constituents. Essentially
it consists of a number of liquid, semi-solid and solid,
colourless hydrocarbons (related to the paraffins), asso-
ASPHALTUM 261
ciated with certain ill-understood dark-brown or black
substances, v/hich constitute the useful part of the raw
material. The best varieties for artistic use are those
which contain the smallest proportion of the above-
described hydrocarbons, for to the presence of these the
treacherous character of asphalt as a pigment is due. On
this account the hardest, most earthy and most brittle
kinds should be chosen, and the crushed samples should
always be submitted to a temperature of at least 250° C.
before being ground in oil or turpentine. The operation
of roasting native asphalt — keeping it over a slow fire
* till it will boil no more and becomes nearly a cinder '
— was recommended by Williams in his ' Essay on the
Mechanic of Oil-Colours' (1787), and furnishes a perfectly
satisfactory and safe product.
If carefully-selected asphalt be submitted to either of
the processes named above, and then be moistened with
spirits of turpentine, and ground in drying-oil (prepared
with borate of manganese), a paint is obtained which
neither cracks nor moves on the canvas like the unpurified
material. Its fixity is further ensured by mixing it with a
little copal varnish, and more particularly by associating
it with a denser pigment, such as umber or flake-white.
It is superior to the imitative asphalts made from coal-
tar, now largely sold in lieu of the original and genuine
product. The disadvantages attending the use of these
coal-tar browns and of ordinary asphalt are two-fold.
Not only are they treacherous on account of their easy
fusibility, but they are liable to stain contiguous pigments
by reason of their solubility in oil or varnish. When
used successfully by the older artists they were always
introduced sparingly, or were largely commingled with
more solid paints.
262 ASPHALT UM
* Mummy,' as a pigment, is inferior to prepared, but
superior to raw, asphalt, inasmuch as it has been sub-
mitted to a considerable degree of heat, and has thereby
lost some of its volatile hydrocarbons. Moreover, it is
usual to grind up the bones and other parts of the mummy
together, so that the resulting powder has more solidity
and is less fusible than the asphalt alone would be. A
London colourman informs me that one Egyptian mummy
furnishes sufficient material to satisfy the demands of his
customers for twenty years. It is perhaps scarcely neces-
sary to add that some samples of the pigment sold as
' mummy ' are spurious. Mummy was certainly used as
an oil-paint at least as early as the close of the sixteenth
century.
Asphalt, after having been heated to drive off the
hydrocarbons previously alluded to, cedes to ammonia a
considerable quantity of a dark-brown colouring-matter,
which in this way may be made available for water-colour
painting. The ammoniacal solution is either evaporated
slowly, to the consistence of a thick syrup, after the addi-
tion of a little gum and glycerin, or it is precipitated with
acetic acid, and the precipitate (after washing) is mixed
with gum and glycerin, and then partially dried until it
has acquired a suitable degree of consistency. But the
water-colour paint thus made is not permanent.
Merimee's process for preparing asphalt for use as an
oil-colour cannot be recommended. He introduces shellac,
white wax, and Venice turpentine into the mixture, as
well as a large proportion of boiled linseed-oil. This
preparation constitutes a very treacherous pigment.
. PR USSIA N BRO WN 263
Prussian Brown.
This pigment, as usually met with in commerce, con-
tains a considerable quantity of a soluble salt of potash,
and is not fitted for the use of artists. But this impurity
need not be present, as it may be removed by a thorough
washing of the powdered colour with boiling water. A
still better plan is to prepare Prussian brown from one
of those varieties of Prussian blue which contain no
potassium. It is made by throwing small pieces (the size
of hazel-nuts) of Prussian blue upon a plate of iron main-
tained at a red heat. Each fragment burns hke tinder,
and if care be taken to employ the right heat for the
proper time, will show a mixed hue, partly yellowish
brown, partly reddish brown, and partly black. The
product, still hot, is thrown into water, ground, washed,
and dried. Prussian brown thus made has a rich
colour of considerable translucency and good drying
character. In hue it is warmer than asphalt. It is quite
permanent.
CHAPTER XIX
BLACK PIGMENTS
Indian Ink : Chinese Ink — Japanese Ink — Encre de Chine
— Chinesische Tusche,
This ink has been prepared in China for at least 2,000
years. It consists essentially of a very fine lamp-black,
associated with gelatin, and scented with musk, camphor,
cloves, or rose-water. The lamp-black employed is
derived from the imperfect combustion of oil or of pine-
wood. The oils chiefly used are those of Sesamum indi-
cum, Cannabis sativa, and Dvyandria cordata ; but in some
factories rape-oil, bean-oil, or the oil of Gleditschia sinensis
is employed. According to the treatise of Chen-ki-suen,
which was written a.d. 1398, these oils are burnt in
small earthenware lamps in the presence of a limited
supply of moist air. The smoke is collected in earthen-
ware conical covers; from these the condensed soot is
removed at short intervals, care being taken to preserve
those portions only which are free from tarry products.
The soot is finally sifted, and reduced to an extremely
fine powder. Lac-resin, rock-oil, as well as many kinds
of wood, have been employed for the preparation of this
carbonaceous basis of China ink in different parts of the
empire, and at different times. It would appear that
from all of these combustibles, if due care be taken, an
264
INDIAN INK 265
excellent product may be obtained. The next step in
the manufacture consists in the incorporation of the soot,
prepared as above described, with the necessary amount
of size. The size employed should be made from a
mixture of clean fish-glue and parchment-size; this is
thoroughly mixed, while warm, with the fine soot. The
paste thus formed is made into balls, which are heated
for some time at the temperature of boiling water, and
then fashioned roughly into sticks. These are subjected
to repeated blows — some hundreds, at least — from a
hammer; they are re-heated occasionally during this
operation, in order to prevent them from becoming hard.
The perfume, consisting of musk and camphor, mixed
with a little rose-water, is incorporated with the mass.
The material, after further pounding and beating, may
then be pressed into the wooden moulds which are to
give it its final form. A tedious process of drying
follows next, the sticks of ink being finally packed in
the ash of rice-straw frequently renewed. After the
removal of adhering ash, the sticks are cleaned, gilt or
otherwise ornamented, and polished with an oily brush.
If the soot used in the manufacture of this ink be con-
taminated with tarry or empyreumatic matters, it yields
a brownish-black product ; the purer and the finer the
state of division of the carbon, the more intense is the
black, and the more neutral are the greys which it yields
on dilution with water, or by admixture with opaque
white pigments.
There are many qualities of this ink made in China.
The best kinds are hard and homogeneous ; they show a
lustrous black fracture, and, when rubbed with water,
do not give rise to the separation of any curdy particles.
Those having, in tint with water, a bluish or violet tinge
266 INDIAN INK
are the most esteemed ; the pure black come next, and
the brownish or yellowish black last.
A stick of Indian ink should be rubbed on the palette,
in straight lines, backwards and forwards. A very con-
venient ink-slab for this purpose forms part of the fittings
of the Japanese writing-case or box, called ' suzuri-bako.'
It consists of an oblong block of dark slate, in which a
sloping cavity has been hollowed out. The slight
* tooth,* or roughness of the surface, greatly helps the
rubbing-down of the ink. A strong wash of this ink, on
sized paper, should not be affected, when once dry, by
having water brushed freely over it. Bluish-black and
pure black Indian ink is permanent under prolonged
exposure to sunlight or impure air ; but the brownish
varieties become paler, less brown, more neutral, by
the action of light. Like most carbonaceous matters,
Indian ink occasionally tends to absorb to a small
extent some of the organic pigments which may be
brought into contact with it in the process of colouring a
drawing.
Indian ink is available for tempera-painting, but can-
not be used with oil.
Indian ink has been imitated with fair success in
France. The chief difficulty in preparing it seems to
lie in the preparation of a suitable gelatinous medium
with which to mix the purified lamp-black. A size
prepared from washed fish-glue (by partial precipitation
with tannin, and subsequent solution of the precipitate
in more size) affords a good binding material. The size,
in true Chinese ink, forms a very large part of the total
weight, varying from one-fourth to over one-third ; but
the European imitations rarely contain as much. Japan-
ese ink is generally inferior to Chinese.
LAMP-BLACK 267
Chinese amateurs sometimes form collections of rare
and old kinds of Chinese ink, occasionally giving for
sticks bearing the marks of renowned makers not less
than their weight in gold. Such specimens are prized
for their rarity, not on account of any special merits
as pigments which they may possess ; and a Chinese
ink collector regards it as an unpardonable offence to
moisten any of his specimens with a view to testing their
quality.
Lamp-Black : Noiv de Lampe — Noiv de Fumee — Noiv de
Houille — Rnss — L ampenschwavtz.
When resins, resinous woods, fatty oils and fats,
paraffin and paraffin oil, or coal-tar oils, are burnt with
an insufficient supply of air, a considerable part of the
carbon they contain may be deposited in the form of soot.
This soot is not, however, pure carbon, but retains
variable proportions of the tarry products of imperfect
combustion or destructive distillation ; these impart to
lamp-black a more or less pronounced warm brownish
hue, except in the cases in which it has been prepared
by processes specially devised to intercept the tarry
and oily products in question. Sometimes small fur-
naces, sometimes large lamps with long wicks are
employed in its manufacture, the soot given off being
collected in two or more receivers ; the soot first de-
posited contains the larger part of the impurities. This
point may be illustrated by the simple experiment of
depressing a white porcelain plate into the flame of a
candle ; the nearer the plate to the wick, the browner
will be the soot deposited.
Lamp-black carefully made is an unalterable pigment ;
268 CHARCOAL-BLACK
but its employment for pictorial purposes has frequently
been objected to by writers on artistic practice as
tending to heaviness and opacity in the shadows.
With the exception of a few pigments of organic
origin, which in water-colour painting have a tendency
to cede some of their colouring matter to any kind
of carbonaceous black, lamp-black exerts no injurious
influence upon any pigments which are stable when used
alone.
Numerous carbonaceous substances have been used as
black or brown-black pigments. Amongst these anthracite
and common bituminous coal may be named. Van Man-
der and De Mayerne both mention forge or pit coal. Black
shales and black slates have long been employed in the
preparation of dark grey pigments. These minerals owe
their colour in part to carbon or compounds of carbon,
in part to very finely divided iron pyrites. They are
permanent, so far as the carbon present is concerned.
Charcoal-Black : Blue-Black — Vine-Black — Frankfort-
Black — Noir de Vigne — Rehschwartz.
When non-resinous woods and woody tissues are
strongly heated in crucibles or other almost completely-
closed vessels, the residue contains the greater part of the
carbon of the original material, and preserves its form.
As the charcoals thus made contain some soluble mineral
matter, particularly potash salts, they should be coarsely
ground and thoroughly washed with hot water previous to
their being finally converted into paints. They are also
hygroscopic, and therefore need to be dried before being
ground in oil.
Amongst the best materials for producing these char-
coal or vegetable blacks may be named the hard shells or
. CHARCOAL-BLACK 269
stones of plums, almonds, and similar fruits; coco-nut shell
broken into small pieces; cork cuttings, and the twigs of
beech and shoots of vine. Coco-nut yields the densest,
most velvet-like black; vine-shoots a less solid black,
having a bluish hue. Washed wine-lees yield a rich black
— Frankfort-black.
When charcoal has been prepared at a sufficiently high
temperature, and then has been thoroughly washed, there
is no question as to its permanence as a pigment in all
media. But if it retains tarry matters, or has been im-
perfectly carbonized, then it may become greyer or less
brown (when used in thin washes) after long exposure to
Hght. This change is due to the oxidation of the tarry
matters, or of the brown substances which are interme-
diate in composition between the original vegetable
materials and the carbon, which is the final product of
their torrefaction out of contact with the air. And one
peculiar property, possessed in varying degrees by char-
coal of every kind, must not be lost sight of. Charcoal
withdraws the majority of organic colouring matters from
suspension, and even from solution in water. A pale tint
of rose-madder in water, if a pinch of coco-nut or vine-
black be added, becomes rapidly decolourized, the pig-
ment being completely absorbed, although not really
destroyed. Animal charcoal (bone-black, ivory-black)
exerts a still more energetic action of this character;
but lamp-black is less powerful.
It is to be regretted that the beautiful bluish blacks,
derived from such materials as the shoots of the vine and
beech, are frequently fraudulently imitated by mixtures of
lamp-black and indigo. The purple vapours of indigotin,
given off when such a mixture is heated in a test-tube,
betray the adulteration at once.
270 IVORY BLACK
Ivory Black : Noiv d'lvoive — Elfenheinschwartz.
Waste ivory, in the form of turnings, shavings, and saw-
dust, if charred in closed vessels, leaves a black residue,
which, however, consists essentially of bone-earth (calcium
carbonato-phosphate), stained by the presence of a few per
cents, of carbon. On exposure to the air, ivory black
absorbs, not only the gases of the atmosphere, but also a
very considerable proportion of moisture. On this account
ivory black, just previous to its being ground in oil, should
always be thoroughly dried. As this substance has a strong
decolourizing power when placed in contact with moist
organic pigments, it is better adapted for use in oil paint-
ing than with water colours. This decolourizing property
belongs also to the animal blacks obtained by charring
horn and bone shavings — products which are frequently
substituted for genuine ivory black. Sir Charles Eastlake
tells us that in a collection of specimens of water-colour
tints inserted in the Mayerne manuscript the 'hartshorn*
black, made from covnu cervinum, remains very intense.
Ivory and bone black are perfectly permanent black
pigments ; the latter is quite free from the bluish tinge
which characterizes most of the vegetable charcoals.
Both are better adapted for oil than for water-colour
painting, for which vine black is preferable.
Black Lead : Plumbago — Graphite — Graphit.
The material of which so-called black-lead pencils are
made is essentially nothing but one of the three forms in
which the element carbon occurs ; diamond is another ;
lamp-black may be taken as a good representative of the
third form. Graphite is the most appropriate name for
the mineral commonly known as black lead, but which
GRAPHITE 271
in reality contains no lead, and is in no way related to
that metal. Graphite was found of good quality at
Borrowdale in Cumberland, a locality which has been
stated, probably erroneously, to be now exhausted ; but
excellent supplies of this mineral come from the Albert
Mine, Siberia, Mount Alibert in the Ural, Ceylon, and
many other places. Graphite, however, is never found
in a state of perfect purity, always leaving, when burnt,
some incombustible matter or ash.
In order to prepare graphite for use as a pigment, the
purest pieces should be selected ; these should be at first
broken into small fragments in order that obviously con-
taminated bits may be rejected before the material is
ground. Inferior qualities of graphite may be purified
by being coarsely ground, and then heated with strong
hydrochloric acid to remove iron and alumina. After
washing, the thus-far purified material should be placed
in a leaden or platinum vessel and gently warmed with
a solution of hydrofluoric acid to remove silica. The
graphite, after this treatment, is to be washed with
abundance of water, and ground to a very fine powder,
which is then ready for admixture with gum water or,
after drying, with oil. The addition of a little china clay
during the grinding of graphite in oil tends to produce a
more manageable paint.
Graphite has a very dark-grey colour, and forms pure
tints of grey with white pigments. It is equally well
adapted for use in water-colour, oil, fresco, and tempera
painting. It is absolutely permanent, and without action
on other pigments.
Graphite is sometimes adulterated with charcoal or
lamp-black. When a very thin layer of the suspected
pigment — freed from oil or other vehicle — is spread on a
272 SEPIA
piece of platinum foil and strongly heated over a lamp^
the lamp-black will burn away, leaving the true graphite
unaltered. If a change of hue or tint occurs, this is a
sign of the presence of the above-named adulterant.
Sepia.
The dark-brown colouring-matter from the ink-bag of
Sepia officinalis, Loligo tunicaia, and other species of cuttle-
fish common in the Mediterranean and Adriatic, has not
been thoroughly investigated from a chemical point of
view. The pure pigment, which constitutes four-fifths
of the weight of the dried ink-bags as they occur in com-
merce, partakes of the character of a weak organic acid,
and is soluble in alkalies and precipitated by acids. In
preparing the substance for artists' use, it is commonly
first removed from the sacs containing it, dissolved in
soda or ammonia solution, and then, after straining the
solution, thrown down by neutralizing the alkali with
hydrochloric acid ; the precipitate is then washed by de-
cantation, and dried. Sometimes the filtered ammoniacal
solution is used directly as a liquid ink. The chief
impurities of the natural sepia are salts of lime and
magnesia, which may be partly removed by washing the
dried and crushed sepia first with dilute hydrochloric
acid and then with water, previous to dissolving it.
Sepia is of a redder or warmer brown hue than bistre,
but it is not so reddish as Vandyke brown. Of these
three pigments, there is no question that sepia is the least
alterable. It is not, however, permanent when exposed
to sunshine, although in darkness or in diffused daylight
it suffers no appreciable change either in depth or quality.
It might, indeed, have been supposed that sepia would
prove unalterable, from the consideration of the observed
SEPIA 273
fact of the pigment from the ink-bags of fossil cuttle-fish
showing now, when used as a water colour, the same
hue and the same chemical characters as recent sepia
from the Mediterranean cuttle-fish of to-day. But it
must be recollected that this fossil pigment has been
excluded completely from the adverse influence of light,
and in great measure from that of air, during the long
ages which have elapsed since it was embedded in rock.
And it is light which, in the case of the majority of
organic pigments, is the determining cause of the changes
brought about by the combined presence of moisture and
atmospheric oxygen.
Sepia is not employed as an oil colour.
There are several grey pigments which, being com-
pounded of other paints already described, need hardly
be mentioned here. Amongst these are neutral tint and
Payne's grey, which in water-colour are still prepared
by commingling indigo, crimson lake, and ivory black,
but which have been replaced in oil by artificial ultra-
marine, ochre and ivory black — an entirely trustworthy
mixture. Slate grey, prepared from a rather soft and
very dark-coloured slate, is a satisfactory pigment both
in artistic quality and in stability.
18
CHAPTER XX
CLASSIFICATION OF PIGMENTS
The painter naturally classes pigments according to their
colour, or more exactly according to what are called the
constants of colour — namely, hue, brightness and purity.
He also takes into account transparency and opacity,
although these terms are conventional and comparative
only, since no pigment is perfectly transparent, none
perfectly opaque. Another basis of classification, and a
very important one, that of stability, will be considered
in the next chapter ; at present we are concerned with
none of these methods of grouping pigments, but with
others founded either upon their origin, or their physical
characters, or their chemical composition.
Pigments are often classified into two groups — the
mineral, and the organic. It is necessary to divide these
groups further, in some such way as this :
( Natural : as ochre, terre verte, ultramarine.
Mineral Pigments, i . ^-c • i ,• • •.• u i*ui
I Artincial : as aureolin, vindian, cobalt blue.
/ Animal : as Indian yellow, carmine, sepia.
Organic Pigments, j Vegetable : as gamboge, sap green, indigo.
' Artificial : as Prussian blue, verdigris.
Such -a classification brings into prominence one
marked distinction between the two groups, for, in
accordance with one's expectation, the mineral pigments
274
CLASSIFICATION OF PIGMENTS 275
are, as a rule, characterized by a much higher degree of
permanence than those of organic origin. [The chemist
looks upon all compounds containing carbon, save the
carbonates, as organic; but at the same time the dis-
tinction between organic and inorganic, or organic and
mineral, is nothing more than a convenient convention.]
Other bases of classification are afforded by physical
or mechanical characters. Pigments may be fixed or
volatile, soluble or insoluble, crystalline or amorphous,
substantive or adjective. There are difficulties in carry-
ing out these schemes of classification, and it will be
found that distinctions of physical character are utilized
to the best purpose when connected with such a chemical
classification as is offered below.
The simplest chemical classification is this :
Elements ; as graphite, lamp-black, gold.
Compounds ; as aureolin, viridian, vermilion.
Mixtures ; as yellow ochre, brown pink, rose madder.
After considerable expansion and some rearrange-
ment, the differences just indicated afford a reasonable
basis for a chemical classification which will prove of
real service in judging of the degrees of stability, and of
the possible interaction of pigments."*^ The mixed and
* In a pamphlet by M. Paul de Lapparent there is an ingeniously
constructed diagram giving, at one view, a list of pigments which,
in the judgment of the author, react upon one another. If we omit
from the list red lead, we shall be able to cancel nearly one-third
of the black marks assigned to mixtures of two pigments ; but we
shall still include two conspicuous offenders, namely, pale cadmium
and Prussian blue. Pale cadmium is stated to be incompatible
with Indian red, Venetian red, burnt sienna, the native earths,
ivory black, Prussian blue, and cobalt violet ; while Prussian blue
is marked as affecting, or as affected by, Venetian red, burnt sienna,
deep cadmium, orange cadmium, pale cadmium, aurora yellow.
276
CLASSIFICATION OF PIGMENTS
even indefinite character of many pigments, and the
chemical solitariness of others, preclude the formation of
groups having equal rank and precise group-characters :
thus the proposed chemical classification, though con-
venient, has no pretension to completeness. I suggest
the following nine groups :
Group I. — Elements
Sih^er.
Gold.
Platinum.
Ivory black
Charcoal black
Lamp black
Indian ink
Graphite
■Contain carbon.
Aluminium.
Zinc white -
Green oxide of
chromium
Burnt umber
Cobalt green
Cobalt red -
Cobalt blue -
Group II. — Oxides
- ZnO. Coeruleum
Red lead - •
Venetian red
Oxides of Fe Light red
and Mn. Indian red
- CoO,«ZnO. Burnt sienna
- CoO.wMgO.
- CoOjHAUOg.
- CoO.nSnOs
- Pb304.
■Chiefly FegOg.
Group III.
Cadmium yellow - • - CdS.
Kings' yellow . . - - AsgSg.
Realgar - AsgSa.
Vermilion HgS.
-Sulphides
Ultramarine
Artificial ultramarine
Green ultramarine
Red ultramarine
the native earths, ivory black, white lead, zinc white, and viridian.
On the other hand, M. de Lapparent allows that vine black is an
entirely innocuous and permanent pigment. My experience is not
in general agreement with the judgments of M. de Lapparent as
embodied in the diagram under consideration. For instance, I do
not find that aureolin reacts with white lead or with ultramarine,
nor can I admit that Prussian blue deserves the bad character
assigned to it. Possibly the purity of the particular pigments with
which our author dealt was not assured in all cases. Might there
not have been free sulphur in his pale cadmium and free acid in his
Prussian blue ?
CLASSIFICATION OF PIGMENTS
277
Yellow ochre
Raw sienna
Raw umber
Group IV.— Hydrates
FeaOg.wHaO.
Oxides and hy-
drates of Fe
and Mn.
Emerald oxidej^, q j^ q^
of chromium;
Mountain blue - CuU^O^.
Flake white
Whitening
Group V. — Carbonates
2PbC03,PbH202.
CaCOo.
Chessylite
Malachite
CuC03,CuH202.
aCuCOg.CuHgOg.
Group VI.
/Silicate of Fe, K,
I Mg.
Egyptian blue CuO,CaO,4Si02.
Terre verte
-Silicates
Smalt
f Silicate of Co
I and K.
Baryta yellow -
Strontia yellow
Chrome yellow
Group VII. — Chromates
- BaCrOj. I Chrome red -
- SrCr04. I Zinc chromate
- PbCr04. I
ZnCrO^.
Group VIII.— Various Inorganic Salts
Baryta white
Lead sulphate
Aureolin -
Naples yellow
BaS04.
PbS04.
KfiCogiaNOg.
Contains Pb,
Sb, O.
o u • r X /"Contains
Schwemfurt greenj ^u, As. O.
_ ^ f Chromium
Tungsten green | t„„g,t^,.
Manganese violet | ^ ,
XT" 1- • 1 .-^ metaphos-
= Nurnberg violet , /
^ I pnate.
Group IX.— Organic Compounds
Indian yellow.
Yellow lake.
Gamboge.
Pure orange.
Rose madder.
Madder carmine.
Rubens madder.
Madder red.
Purple madder.
Brown madder.
Scarlet alizarin.
Crimson alizarin.
Carmine.
Crimson lake.
Scarlet lake.
Purple lake.
Sap green.
Verdigris.
Emerald green.
Indigo.
Prussian blue.
Antwerp blue.
Bitumen.
Bistre.
Sepia.
Vandyke brown B.
278 SULPHIDES
One of the chief lessons to be learnt from this
classification is this, that the members of each class, as
a general rule, exert no action upon one another. This
is explained easily. The oxides of Group II., having
already taken up the full complement of oxygen which
they can acquire under ordinary conditions, are not likely
to be oxidized by admixture with other oxides of similar
character. In the same manner the sulphides of
Group III. neither give sulphur to, nor receive it from
the other sulphides, for all but one of them have been
produced in the presence of excess of sulphur. The
following characteristics of each group may prove useful
in the study of their chief members :
Group I. : Elements. — All the black pigments in
ordinary use consist of or contain the element carbon,
and are not subject to change : graphite is a form of
carbon and is unalterable and inert. Gold, if pure or
nearly free from alloy, is not liable to chemical change,
but silver readily tarnishes by combining with sulphur ;
drawings in silver-point are frequently found to have
altered in hue from this cause. The best metal-point for
fine drawing is pure platinum.
Group II. : Oxides. — These have generally been
prepared at a high temperature, and are not easily
amenable to chemical or physical change : they are,
moreover, not liable to affect other pigments, being
practically inert.
Group III. : Sulphides. — Some of these may give up
sulphur to the metallic bases of other pigments. Thus
cadmium yellow blackens emerald green, producing
copper sulphide. One of these pigments, vermilion, is
prone to a molecular change, whereby the red crystalline
form passes, without chemical alteration, into the black
amorphous variety. The members of this group some-
VARIOUS INORGANIC SALTS 279
times contain free sulphur, or injurious sulphur com-
pounds.
Group IV. : Hydrates. — The water present in these
compounds exists in two states, essential and hygro-
scopic. Sometimes a part of the former may be lost, and
a change of hue occur in consequence, but the alteration
is rare, save in the pigments which owe their colour to
the presence of copper hydrate. In the case of raw
umber, the water present acts rather in aiding the oxygen
of the air, under the influence of sunHght, to oxidize
some of the peaty or bituminous matter sometimes
present in this pigment.
Group V. : Carbonates. — Three out of the four
carbonates included in this group are liable to suffer
change on account of the metal they contain (lead or
copper) combining with sulphur, and so forming a brown
or black sulphide.
Group VI. : Silicates. — These are generally inert
bodies little prone to suffer or cause change. Some of
the ochreous earths contain silicates of iron, manganese,
and alumina, as well as the hydrates of the two former
metals, and so might be placed in this group.
Group VII. : Chromates. — This group consists of
compounds rich in oxygen. When in contact with some
of the more alterable organic pigments belonging to
Group VIII. the chromates occasionally lose oxygen.
On this account they show a tendency to acquire a
greenish or greyish hue, the result of the reduction of
the red oxide of chromium to the green oxide. This
change is accompanied by a corresponding injury to the
colour of the organic pigment which has been involved
in the reaction.
Group VIII. : Various Inorganic Salts. — A num-
ber of miscellaneous compounds having no chemical
28o
CLASSIFICATION OF PIGMENTS
relationship have been thrown into this group. One, a
sulphate, is insoluble and inalterable ; another, potassium
cobalti-nitrite, is generally inert, but, owing to its nitrous
constituent, acts injuriously upon some organic pigments,
notably on indigo. The members of the group which
contain lead or copper may darken in the presence of
sulphuretted hydrogen and of some other sulphur com-
pounds.
Group IX. : Organic Compounds. — This group in-
cludes many more pigments than any other : not one of
its members possesses the permanency belonging to the
majority of the mineral pigments, while some are so
fugitive that they may even be used for producing a
photographic picture by being exposed to sunlight under
a negative. This fading is generally due to the com-
bined action of water and oxygen : in oily and resinous
media it is lessened, retarded, or even prevented by the
hydrofuge character of these vehicles.
It should be noted that members of each group,
though presenting one or more characters in common,
often exhibit certain chemical and physical differences of
deportment. Here is a list of the chief changes which
they are capable of suffering, with illustrative examples :
Molecular rearrangement
Subsidence
Volatilization
Solution ...
Fusion
Oxidation ...
Reduction . . .
Sulphuration
VermiHon ; cadmium sul-
phide.
Smalt.
Kings' yellow.
Strontia yellow ; aureolin.
Asphalt.
Carmine ; Vandyke brown.
Naples yellow ; chromates.
White lead ; emerald green.
CLASSIFICATION OF PIGMENTS 281
The effect of pulverization upon pigments may be
mentioned in this connexion. Generally, the more
finely an alterable pigment is ground, the more suscep-
tible does it become to chemical injury : its colour
becomes at the same time paler, and may even change
in hue as well as in tone. Continued grinding, beyond
the degree necessary to develop the proper colour, im-
proves some pigments, but injures others.
In the chemical classification of pigments which has
been sketched in the present chapter there comes out in
rather striking relief one point to which no allusion has
yet been made. It is indeed a point which touches the
chemist rather than the artist, and yet it seems to merit
a passing reference in this place. I refer to the number
of gaps in the table — to the number of elements and of
classes of salts which have no place therein. The
scientist will not, indeed, expect to find amongst these
9 groups any substances possessed, to a marked degree,
of the property of solubility in water, for he will recog-
nise the unfitness of such bodies for use as paints. But
he will probably observe, with some surprise, that there
are no compounds of fluorine, chlorine, bromine, or iodine
in our lists, but two sulphates, only one substance con-
taining tungsten, and not one containing nickel, uranium,
molybdenum, or vanadium. Coloured insoluble com-
pounds indeed exist of all these elements ; several of such
compounds have been proposed and even employed as
pigments, but instability or injurious action upon other
pigments has banished these candidates from the palette
of the artist.
A few words may be added here as to the distinction
between substantive and adjective pigments. Of the
former class aureolin, vermilion and malachite may be
282 CLASSIFICATION OF PIGMENTS
cited as examples ; to the latter all the lakes belong.
Substantive pigments are homogeneous, definite sub-
stances, simple or compound, but not separable into two
bodies, having differing optical qualities. On the other
hand, adjective pigments contain two substances, one
coloured, and another generally colourless, through which
the former is disseminated or over which it is extended
as on a background.
CHAPTER XXI
TABLES OF PERMANENT, FUGITIVE, AND ALTERABLE
PIGMENTS
By several different methods, data may be obtained which
enable us to classify pigments — roughly, it is true — in
accordance with their varying degrees of stability. Such
data are derived partly from the known chemical and
physical constitution of the various substances; partly
from a study of old paintings and drawings in which they
have been used ; and partly from special experimental
tests of permanency to which they have been subjected.
Selections from these data are given in Chapters XX.,
XXIV., and XXVI., of the present work ; but much
additional information has been furnished by other trials,
conducted by the author and other experimenters, for
which space could not be found in this volume. Tables
constructed from such data must not be regarded as
affording exact values, but merely approximations. From
some minute and often obscure cause differences of deport-
ment, under exposure to hostile influences, will occasion-
ally be observed in the case of two specimens of the same
pigment having the same hue. And, further, the group-
ing of pigments into a small number of classes is a
conventional and convenient arrangement which cannot
accurately represent the numerous degrees of stability or
instability which characterize the several pigments under
283
284
TABLES OF PIGMENTS
discussion. For when we leave the practically unalter-
able mineral pigments, we have to deal with a number of
preparations which fall by irregular and often barely recog_
nisable steps from the almost permanent to the hopelessly
fugitive. One example of this difficulty in classification
must suffice : aureolin is almost worthy of a place in
Class I., Indian yellow scarcely deserves inclusion in
Class II. The action of mixed pigments upon one another,
though not as frequent as it is supposed to be, creates
another difficulty in our classification, so also does the
medium employed in painting, which may either protect
an alterable pigment from change or aid in its destruction.
In fact, each method of painting, if really distinct, requires
a special classification of the pigments to be employed in
carrying it out.
In the annexed classification, a limit of three orders of
stability has been adopted, the first class including the
practically permanent pigments ; the second class those
which, though liable to a variable measure of change, may
yet generally be allowed ; and the third class those which
should be definitely excluded from the palette :
CLASSIFIED TABLE OF PIGMENTS FOR OIL-PAINTING
Class I Class II Class III
Baryta white.
Zinc white.
Flake white.
Yellow ochre.
Raw sienna.
Baryta yellow.
Naples yellow.
Cadmium orange.
White
Yellow
Aureolin.
Indian yellow.
Strontia yellow.
Chrome yellow.
Cadmium yellow.
Kings' yellow.
Yellow madder.
Brown pink ; yellow
lake.
Gamboge.
Zinc chromate.
TABLES OF PIGMENTS
28s
Class I
Vermilion.
Indian red.
Light red.
Venetian red.
Red ochre.
Cobalt violet.
Mars violet.
Violet ultramarine.
Emerald oxide of
chromium.
Green oxide of chro-
mium.
Cobalt green.
Green ultramarine.
Ultramarine.
Artificial ultra-
marine.
Cobalt.
Coeruleum,
Burnt sienna.
Raw and burnt um-
ber.
Cappagh brown.
Verona brown.
Prussian brown.
Vandyke brown A.
(earthy).
Ivory- black.
Charcoal-black.
Lamp-black.
Graphite.
Class II
Red
Madder carmine.
Rubens' madder.
Rose madder.
Madder red.
Purple madder.
Scarlet lake (ali-
zarin).
Violet
Manganese violet.
Green
Emerald green.
Terre verte.
Malachite.
Madder green.
Blue
Smalt.
Prussian blue.
Antwerp blue.
Chessylite.
Brown and Black
Madder brown.
Cologne earth.
Class III
Crimson lake.
Carmine and burnt
carmine.
Indian lake.
Scarlet lake (cochi-
neal) .
Purple lake.
Violet carmine.
Verdigris.
Sap green.
' Green vermilion,
etc.
Green verditer.
Indigo.
Blue verditer.
Blue ochre.
Vandyke brown B.
{bituminous).
Bistre.
Sepia.
Bitumen ( = asphalt).
286 TABLES OF PIGMENTS
In order to adapt the foregoing classified table to water-
colours, some changes and additions must be made.
Flake white, Naples yellow (true), cadmium (pale), and
vermilion (artificial), must be removed from the Class (I.)
of permanent pigments and placed in Class III., to which
also must be relegated several pigments from Class II.,
namely, chrome yellow, malachite, and madder brown.
Of course, it should be clearly understood that no pigment
belonging to Class III. should be employed in artistic
painting. One satisfactory addition, and one only, can
be made to Class I. in the table. Indian ink is a pigment
available for water-colour painting, and when it is free
from a brownish hue may be safely used. Bistre and
sepia are likewise used only as water-colours, but they are
both fugitive, and must be placed in Class III. Almost
the same modifications of the table are required in the
case of tempera-painting as in water-colour painting.
With fresco-painting the exclusion of many more pigments
is an absolute necessity, as they are completely ruined by
caustic lime. Not only are all the chromates inadmissible,
as well as all the pigments which cannot be trusted as
water-colours, but likewise Prussian blue and Antwerp
blue, while the madder colours are much altered in hue
when used in this process. In stereochromy the number
of available pigments is still further reduced.
It may not be uninstructive if we cite in this place the
classification of pigments as used in oil which M. Decaux
has published. The order followed by this experimenter
is that of stability ; the figures prefixed to the names of
the individual pigments indicate the degree of perma-
nence, I marking out the materials which are quite
unchangeable, while 45 is the most fugitive of all :
TABLES OF PIGMENTS
287
DECAUX'S TABLE OF PIGMENTS FOR OIL-PAINTING
I. Zinc white.
I. Flake white.
I. Yellow ochre.
I. Naples yellow.
I. Cadmium (deep).
I. Raw sienna.
I. Red ochre.
I. Mars red.
I. Venetian red,
I . Burnt Italian earth.
Class I
I. Green oxide of chro-
mium.
I. Ivory black.
I. Terre verte,
I. Green ultramarine.
I. Cobalt blue.
I. Artif. ultramarine.
1. Ivory black.
2. Mars brown.
3. Burnt sienna.
4. Cobalt green.
5. Mars yellow.
6. Mars orange.
7. Burnt umber.
8. Viridian.
9. Indian red.
10. Mars violet.
1 1 . Indian yellow.
12. Emerald green.
13. Malachite green.
14. Scheele's green,
15. Raw umber.
16. Vandyke brown.
Class II
17. Prussian blue. 26. Madder ' rose
18 to 23. Various madder doree. '
lakes. 27. Brown madder.
24. Madder carmine. 29. Cassel earth.
Class III
30.
Pale chrome.
35.
Asphalt.
43.
Yellow lake.
31.
Zinc chromate.
36.
Brown pink.
44.
Carmine.
32.
Pale cadmium.
38.
Vermilion.
45-
Crimson lake.
33-
Orange chrome.
42.
Burnt carmine.
On comparing this classified list with that previously
given a general accordance will be perceived, the low
position given to raw umber and to vermilion, as well as
the very high place assigned to Indian yellow and to
terre verte, constituting the chief exceptions.
In closing this chapter it may be useful to state that
the pigments to which a place in our Class I. has been
assigned have stood the very severe test of long exposure
to direct sunlight. On a subsequent page it will be shown
that this method of determining the stability of pigments
TABLES OF PIGMENTS
is not in all cases a fair one, because changes brought
about by such exposure may not occur at all when the
temperature does not rise beyond a particular point, and
when the radiant energy of light and actinism does not
exceed a moderate measure of intensity. So far, then, as
exposure to light is concerned, it may happen that some
of the pigments in Class II. really deserve a higher posi-
tion than that assigned to them in our table. In this
connexion we may give some of the conclusions which
Messrs. Winsor and Newton have published as to the
stability of oil colours when exposed, not to sunshine, but
to a strong north light. It will be noticed that the class
of permanent pigments has been greatly enlarged as the
result of the milder ordeal through which the materials
have passed :
Class I. — Permanent
Zinc white.
Aureolin.
Cadmium yellow.
Yellow ochre.
Raw sienna.
Baryta yellow.
Mars yellow.
Vermilion.
Venetian red.
Light red.
Indian red.
Alizarin lakes.
Crimson madder.
Madder carmine.
Pink madder.
Rose madder.
Purple madder.
Brown madder.
Rubens' madder.
Scarlet lake (new).
Burnt lake (madder).
Cobalt green.
Oxide of chromium.
Viridian.
Cerulean blue.
Cobalt blue.
Ultramarine.
Manganese violet.
Cobalt violet.
Prussian brown.
Caledonian brown.
Cappagh brown.
Burnt umber.
Burnt sienna.
Vandyke brown.
Bone brown.
Black lead,
Blue black.
Ivory black.
Class II. — Moderately Permanent
Flake white.
Chrome yellow.
Naples yellow (imi-
tative).
Kings' yellow.
I Indian yellow.
Green cinnabar.
Emerald green.
Malachite.
Rose doree.
Leitch's blue.
Prussian blue.
Antwerp blue.
Asphaltura.
Brown pink.
TABLES OF PIGMENTS 289
Class III.— Fugitive
Citron yellow Burnt carmine. j Green lake.
(ZnCr04). Crimson lake. Sap green.
Yellow lake. Indian lake. Verdigris.
Gamboge. Purple lake. Indigo.
Primrose yellow. Violet carmine. Italian pink.
Carmine. I
19
CHAPTER XXII
SELECTED AND RESTRICTED PALETTES
It is by no means easy to construct a palette which shall
be at once artistically and scientifically perfect. For it
is impossible to exclude every pigment which is suscep-
tible of change, and it is unwise to include every pigment
for which the fancies and partialities of particular painters
desire to find a place. An artist discovers how to obtain
a required hue by means of a special pigment, and is
naturally reluctant to learn by tedious experimenting
whether it cannot be secured by means of a more complex
commingling of the ordinary paints. And although some
great masters have done marvellous things with five, four,
or even three pigments only, there is no sound argument
which can be urged in favour of so severe a restriction.
If much mixing of paints be bad, then a reasonable en-
largement of the palette will render such mixing unneces-
sary. And the artist wants something more than a mere
match in hue : he knows that there is a peculiar quality
of colour to be sought as well. He can make a trans-
parent pigment opaque, but the reverse operation is im-
practicable. Scumbling of one opaque colour thinly over
another which is also opaque very imperfectly attains the
effect of translucency. So the artist demands, in addition
to a chromatic series of opaque pigments, a second series
290
SELECTED PALETTES 291
possessed of transparency, or, at least, of translucency.
Thus he adds to his cadmium yellow, aureolin ; to his
vermilion, madder carmine ; to his emerald green, virid-
ian; to his coeruleum or cobalt, ultramarine. And,
moreover, he has to take account of the peculiar and
often unexpected effects produced by the lightening of
the tone of a pigment by commixture with white, and by
the darkening due to the addition of black. Two nearly
identical translucent reds may yield with white two dif-
ferent hues, one verging on salmon, the other on rose.
Charcoal-black yields with aureolin or Indian yellow a
series of greens quite distinct from those obtained by
mixing these yellow pigments with ivory-black. So the
artist in making his first choice from the whole number
of trustworthy pigments at his command, will proceed
towards his final selection by two stages. He first re-
tains those pigments which commend themselves to his
judgment for their own chromatic qualities when un-
mixed ; he then proceeds to test the characteristics of
the remainder by trying the tints which they severally
produce with white, the shades they yield with black,
and the mixed hues to which they give rise by commix-
ture with one another in twos and threes. To this set of
experiments he adds another, in which these pigments
are mixed, after the same manner, with those belonging
to the first series. As the result of these trials the artist
will be enabled to exclude several paints which would
merely serve to encumber his palette.
Before deciding finally as to the elements which shall
be retained for our fundamental palette, it will be in-
structive to study the selections of pigments which from
time to time have been employed by artists of recent
times and of the present day. The obvious weakness of
292
SELECTED PALETTES
many of such palettes lies in their inclusion of a few
treacherous pigments, such as asphaltum, and of a few
evanescent pigments, such as carmine, crimson lake, and
the bituminous variety of Vandyke brown. Nevertheless,
in making our selection of pigments from the classified
list previously given, we may obtain many useful hints
from the palettes employed by artists with whose works
we are familiar. It is particularly interesting to observe
how extremely restricted were the sets of pigments used
by several painters who are distinguished for the refine-
ment and for the rich variety of hues shown in their
works. In the following paragraphs the names of all
decidedly fugitive and alterable pigments are printed in
italics.
Sir Joshua Reynolds, although too fond of varying his
practice by the introduction of many dangerous com-
pounds, and by the use, in the same picture, of incom-
patible media and methods, executed many works between
the years 1770 and 1775 with one or other of these five
restricted palettes, containing from four to eight pig-
ments :
i. Flake white.
Yellow ochre.
Lake.
Ultramarine.
Black.
ii. Flake white.
Yellow ochre.
Orpitnent.
Lake.
Caroline.
Ultramarine.
Black.
Blue black.
iii. Flake white.
Yellow ochre.
Naples yellow.
Cartnine.
Vermilion.
Ultramarine.
Black.
iv. Flake white.
AspJialttcin.
Vermilion.
Blue.
V. Flake white.
Naples yellow.
Lake.
Miniujii.
Asphaltum.
Paul Delaroche and H. Vernet employed these eleven
pigments :
Flake white. Yellow ochre.
Naples yellow.
Raw sienna.
Vermilion.
Lake.
Brun rouge.
Burnt sienna.
Artificial ultra-
Blue black.
Ivory black.
SELECTED PALETTES
293
W, Etty, R.A., used twelve pigments :
Flake white. Naples yellow.
Yellow ochre.
Vermilion.
Light red.
Indian red.
Lake.
Terre verte.
Blue verditer.
Raw umber.
Burnt umber.
Black.
Samuel Palmer employed in oil painting the following
pigments, twenty-eight in all :
Flake white. Naples yellow. Field's vermilion. Ultramarine. Vine black.
Yellow ochre. Vermilion. Ultramarine Ivory black.
Raw sienna. Light red. ash. Broivn madder.
Cadmium i, 2, 3. Venetian red. Cobalt. Raw umber.
Aureolin. ' Indian red. Antwerp blue. Burnt sienna.
Madder carmine. Terre verte.^
Pink madder. Green oxide
Rose madder. chromium.
Emerald green.
Thomas Wright J of Derby, employed fourteen pigments,
and, it is to be presumed, flake-white also :
Naples yellow.
Vermilion.
Carmine.
Terraceum blue.
Ivory
Brown pink.
Burnt ochre.
Lake.
Ultramarine.
black.
Indian red.
Burnt lake.
Prussian blue.
Light red.
Lake azure (?).
From the Portfolio of 1875-6 we obtain the particulars
given below concerning the pigments used by several
well-known artists : the palettes quoted have been chosen
as representative of different types.
P. H. Calderon, R.A., employed fifteen pigments :
Flake white. Naples yellow. Vermilion. Cobalt blue. Burnt sienna.
Yellow ochre. Venetian red. Antwerp blue. Raw umber.
Cadmium yellow. Pink madder. Vandyke brown.
Raw sienna. Ivory black.
Mars yellow.
W. C. T. Dobson, R.A., ten pigments :
Flake white. Yellow ochre. Vermilion. Cobalt blue. Raw umber.
Raw sienna. Rose madder. Vandyke brown.
Purple lake. Ivory black.
The following are water-colour palettes :
Alfred W. Hunt, seventeen pigments, and in addition
Chinese white :
Lemon yellow. Vertnilion.
Gamboge. Light red.
Yellow ochre. Indian red.
Raw sienna. Madder lake.
Terre verte.
Cobalt.
Ultramarine.
Ultramarine ash.
Smalt.
Madder brown.
Raw umber.
Burnt sienna.
Burnt umber.
294
SELECTED PALETTES
Sir John Gilbert, R.A., fifteen pigments :
Chinese white. Yellow ochre.
Raw sienna.
Vermilion,
Light red.
Venetian red.
Indian lake.
Cobalt.
Artificial ultra-
marine.
Indigo.
Prussian blue.
Antwerp blue.
Burnt sienna.
Vandyke brown.
Ivory black.
The selection of a good set of permanent or fairly per-
manent pigments must depend to some extent upon the
idiosyncrasy of the artist, upon his training and methods
of work, upon the class of subjects with which he deals.
As a good general working set for oils, the following selec-
tion is offered. It is arranged in two sections, the second
including what may be called * supplementary ' pigments :
c^„^: T r Flake white.
;n.l H.I' J Cadmium yellow.
. ^'^Z. 1 Aureolin.
12 pigments, (^y^u^^ ^^^^^^
/"Raw sienna.
Section II. I Naples yellow,
includes -! Baryta yellow.
12 pigments. I
Vermilion.
Madder car-
mine.
Light red.
Purple madder.
Madder brown.
Cobalt violet.
Viridian.
Artificial ul-
tramarine.
Green oxide
chromium.
Terre verte.
Cobalt green,
light.
Raw umber,
Cappagh brown.
Ivory black.
Cobalt.
Prussian blue
(insol.).
Burnt sienna.
Emerald green is excluded, since it cannot be safely
associated with cadmium yellow, but there is no reason
why several more pigments should not be added in Sec-
tion II., other than the desirability of limiting the number
of paints to those really required. Garance doree, Rubens'
madder, deep cobalt-green, burnt umber, Verona brown,
vine black, and graphite might be added to the hst. On
the other hand, further restrictions become by practice
possible. One does not know what white, vermilion,
yellow, and vine or charcoal black can do until one has
purposely debarred one's self from the employment of
any other coloured pigments. Here are two such re-
stricted palettes :
I. Flake-white, yellow ochre, light red, cobalt, ivory-
black.
RESTRICTED PALETTES 295
2. Flake-white, cadmium yellow, vermilion, ultra-
marine, ivory-black.
A third restricted palette, containing ten pigments
instead of five, is thus constituted :
3. Flake-white, yellow ochre, cadmium yellow, aureolin,
vermilion, madder carmine, ultramarine, viridian, Cap-
pagh brown, ivory-black.
It is scarcely necessary to say that the capacity of No. i
for representing the range of natural hues is extremely
limited ; indeed, it is fitted only for ' dead colouring,' and
for the ' first painting.' With No. 3, however, we can
imitate with a near approach to exactness all the pigments
excluded from this palette, and we may therefore regard
it as practically complete. Some of the hues obtained by
the mixtures which it is necessary to employ for this pur-
pose will be a little less luminous than the originals, since
these hues will have been produced by the increased ab-
sorption of certain elements of the incident white light —
they are consequently duller, or have more grey in them.
This palette. No. 3, is nearly the same as one devised by
the late Mr. P. G. Hamerton (Portfolio, 1876, p. 132), which
was constituted of flake-white, pale cadmium, yellow
ochre, vermilion, rose madder, artificial ultramarine,
emerald oxide of chromium, Vandyke brown, black. I
have added one pigment, aureolin, and have substituted
for pale cadmium, full cadmium yellow ; for rose madder,
the more stable madder carmine ; and for Vandyke brown,
Cappagh brown. Mr. Hamerton tested the range of his
restricted palette by imitating with its constituents many
of the excluded pigments. I give some of his results, as
modified by my own experiments with my palette No. 3.
Naples Yellow. — Imitated by flake-white, with cadmium
yellow and a trace of yellow ochre : exact.
296 RESTRICTED PALETTES
Lemon Yellow. — Flake-white, cadmium yellow, with a
trace of viridian : less brilliant than the original.
Cadmium Orange. — Cadmium yellow, with vermilion •
less brilliant.
Light Red. — Vermilion, yellow ochre, Cappagh brown.
Venetian Red. — Vermilion, yellow ochre, madder car-
mine, a little Cappagh brown : exact.
Indian Red. — Vermilion, trace of yellow ochre, madder
carmine, ivory black : a good match, but less translucent.
Cobalt Blue. — Artificial ultramarine, flake- white, a little
viridian : less translucent ; does not match cobalt blue by
artificial light.
Prussian Blue. — Ultramarine, black, a trace of viridian :
lacks the translucency and depth of the original.
Raw Sienna. — Yellow ochre, aureolin, Cappagh brown.
Burnt Sienna. — Madder carmine and Cappagh brown,
with a trace of vermilion : less translucent.
Emerald Green. — White, cadmium yellow, viridian,
artificial ultramarine : not so brilliant as the original.
Malachite. — White, cadmium yellow, yellow ochre,
viridian, ultramarine.
Terve Verte. — White, aureolin, viridian, ivory-black.
Cobalt Green. — Ultramarine, viridian, trace of flake-
white.
Indigo. — Ultramarine, with black and trace of viridian :
very close.
Vandyke Brown. — Cappagh brown, with much madder
carmine and a little ivory-black.
It is needless to multiply further our illustrations of the
resources at the command of the painter who limits him-
self to our restricted palette of ten pigments (No. 3,
page 295), as experimental trials of its capacity are
easily made.
RESTRICTED PALETTES 297
So far, then, as regards selected and restricted palettes
of oil colours. Some modifications must be made in our
list in order to devise corresponding palettes of useful
and enduring water-colours. In the more extended list
(p. 294), zinc-white must replace flake-white, while
vermilion, purple madder, brown madder, and cobalt
violet must be discarded. In the limited palette (No. 3),
the changes to be made comprise the substitution of
zinc-white ( = Chinese white) for flake- white, the replace-
ment of vermilion by one of the brightest native varieties
of iron reds (the mineral turgite is perhaps the best kind),
Cappagh brown by Mars brown, and of ivory-black by
Indian ink. The two palettes (A. and B.) will then
finally assume the following forms for water-colours :
(A.) ( Zinc white. Light red. Viridian. Raw umber.
Section I. J Cadmium yellow. Indian red. Artificial ultra- Burnt sienna,
includes | Aureolin. iNIadder car- marine. Indian ink.
13 pigments. \ Yellow ochre. mine. Cobalt.
Section II. ) Raw sienna. Red ochre. Prussian blue Mars brown,
includes V (insol.). Ivory black.
5 pigments. J
Doubtless artists will especially miss from this palette
six pigments, namely, gamboge, vermilion, rose madder,
brown madder, Vandyke brown, and indigo. But after
the overwhelming evidence adduced in Chapter XXVI. as
to the want of permanence shown by these water-colour
paints, one feels compelled to exclude them. Our second
and more restricted palette (B.) is thus composed :
(B.) Chinese Yellow ochre. Red ochre. Ultramarine, Mars brown,
white. Cadmium orange. Madder car- Viridian. Indian ink.
Aureolin. mine.
Although it is obvious that with these limited palettes
it is impossible to produce exact imitations of every ex-
cluded pigment, yet there are two considerations which
must not be forgotten in estimating the influence of this
298 RESTRICTED PALETTES
defect on artistic painting. Foremost may be placed the
fact that pigments are rarely employed wholly unmodified
by admixture with others ; then it must be noted that the
differences between our imitations and the original pig-
ments which they are intended to replace are rather those
of lessened brightness, translucency, and depth than those
of hue.
PART IV
METHODS AND RESULTS
Chapter XXIII.— Painting Methods. Chapter XXIV.— Study of
Old Paintings and Drawings. Chapter XXV. — Conservation
of Pictures. Chapter XXVI.— Trials of Pigments.
CHAPTER XXIII
PAINTING-METHODS
As the grounds, vehicles, and pigments employed in
painting have been already described in Parts I., II.,
and III. of this volume, it will not be necessary to do
more in the present chapter than give a summary or
general view of the chemistry of each method of employ-
ing these materials.
These methods are six in number, and may be thus
defined :
Methods
1. Tempera
2. Fresco
3. Stereochromv -
4. Oil - Painting)
AND Spirit- v
Fresco )
5. Water-Colour -
6. Pastel, \
Charcoal, I_
Plumbago, j'
Silver-Point '
Vehicles
Egg-yolk emulsion ; solution of gelatin
or albumen
Lime-water, in both buon' fresco and
fresco secco
Aqueous solutions of alkaline silicates -
Oil, and solutions of resin, wax, paraffin
Aqueous solutions of gum, glycerin,
honey ......
None
Changes
during Fixing
j Desiccation or
\ coagulation.
Carbonation.
Formation of in-
soluble silicates.
{Oxidation,
Resinification,
Evaporation,
Solidification.
Desiccation.
None.
I. Tempera-painting, or painting in distemper, is gener-
ally assumed to include two, if not three, methods of
procedure, in which different vehicles or media are em-
ployed. These vehicles all contain a nitrogenous con-
stituent ; but in one of them — and that the most important
301
302 TEMPERA -PA INTING
— oil or fat is present in addition. Tempera-grounds
must be rigid, tenacious, and firm; they need not be dry,
but if organic pigments are to be used, they should not
contain caustic lime. Thus, a surface of plaster made
with slaked lime and sand must have been so long
exposed to the air as to have absorbed the amount of
carbonic acid necessary to convert the hydrate of lime
present into ' mild lime ' — that is, the carbonate. To
detect the existence of caustic lime in such a painting-
ground recourse may be had to test-papers. Three kinds
are available for this purpose. Thus, yellow turmeric-paper,
first wetted and laid upon the surface of the plaster,
should show no change of colour ; if it become reddish,
the presence of caustic lime is indicated. Under the
same circumstances red litmus-paper turns blue or purple,
while phenolphthalein-paper acquires a crimson hue. If
these tests show the absence of caustic lime, the painting
may be commenced, otherwise the surface must be car-
bonated by syringing it or washing it with water charged
with carbonic acid gas. These precautions are, of course^
unnecessary in cases where the painting-ground has been
prepared with plaster -of- Paris or other neutral com-
positions of which caustic lime is not a component.
Before commencing work the painting-ground must be
slightly and uniformly moistened with distilled water,
and then coated with weak size. The pigments to be
employed are those recommended for use as water-
colours; they are thoroughly mixed with the medium
to be employed, namely, egg-yolk emulsion, or size, or
prepared white of e^g. These media serve not only to
bind the pigments to the ground, but also the coloured
particles to one another. To render the egg-yolk more
tractable, its alkaline reaction should be exactly neutral-
TEMPERA -PA INTING 303
ized by the cautious addition of a very few drops of white
vinegar — fig-tree sap or white wine was sometimes
formerly employed for the same purpose. Some artists
content themselves with diluting the egg-yolks with a
little water, others add a small proportion of white of egg,
previously shaken with a little water and filtered. To
keep the egg-emulsion sweet, a lump of camphor or a
few cloves may be put into it. Size and also white of
egg have been employed in tempera-painting. The white
of egg needs dilution with water, thorough shaking, and
then filtering through muslin. When egg-yolk is used
in this method of painting, the oil in it gradually hardens,
while the albuminoid matters which accompany it be-
come partly insoluble and coagulated. As the amount
of oil in egg-yolk is twice as great (31 per cent.) as the
albuminoid matters (15 per cent.), this vehicle presents
considerable resemblance to those employed in oil-
painting, the albuminoid matters corresponding in a
measure to the resins often used in the latter method.
This vehicle does not act so effectually as oil and varnish
in ' locking up ' pigments, and so the protection against
change which it affords is less. Moreover, instances have
been observed in which the sulphur present in the albu-
minoids of egg-yolk has acted injuriously upon some of
the pigments of the picture ; but by excluding, as we
now do, all paints containing lead and copper from the
tempera-palette, accidents of this kind are prevented. A
finished tempera-picture was often — one might almost
say generally — rubbed with a cloth and then varnished,
the varnish being often made by dissolving sandarac in
oil. The tone of the colours was thus warmed, while
further protection was at the same time afforded against
moisture and impure air.
304 FRESCO-PAINTING
2. In fresco-painting — both buon' fresco and fresco secco
— the ground must not only be wet, but caustic. In true
fresco the pigments are appHed to the last and freshly-
spread coat of plaster before it has had time to absorb
more than a trace of carbonic acid from the air; the
painting-ground is in fact saturated with an aqueous
solution of hydrate of lime, while there remains a large
reserve of this compound in an undissolved condition.
When on such a surface a layer of pigment mixed with
water is placed, as that water evaporates the lime-water
in the ground diffuses into the paint, soaks it through
and through, and gradually takes up carbolic acid from
the air, thus producing carbonate of lime, which acts as
the binding material in this method. As there still exists
an ample reserve of hydrate of lime in the ground, wetting
the painted surface with pure water will cause more of
this hydrate to enter into solution, and so the liquid
present in the plaster will be reinforced with a fresh
supply of the binding material. Ultimately the ground
and the pigment become incorporated and harden together.
If more binding material be required, it may be intro-
duced by means of lime-water itself, or even by baryta-
water, which contains about twenty times as much
hydrate of baryta as the strongest lime-water contains of
hydrate of lime ; these liquids or hydrate of lime may
also be mixed with the pigments used. Although the
chief binding material in fresco-painting is this carbonate
of lime, yet with some plasters and with some pigments
another substance is produced. This is silicate of lime,
produced by the action of caustic-lime in solution upon
the soluble silica of the plaster or of the pigments.
Some sands, infusorial earths, and ochreous pigments,
contain such soluble silica, but it is certainly not present
FRESCO-PAINTING 305
in every case. Silicate of lime as a binding material is
more permanent than the carbonate.
In fresco secco the plaster is allowed to harden, and, in
some measure, to dry, and the operation of painting may
be continued at leisure. The ground immediately before
beginning work is moistened with lime- or baryta-water,
and the pigments are mixed with one or other of these
liquids, or with a little slaked lime. This modified pro-
cess is far easier of execution than true fresco ; but the
fixation of the pigments, though resulting from the same
cause, is less complete.
In the treatises of Cennini and other later writers the
expression ' painting in secco ' is generally employed to
designate any process of tempera-painting, but the fresco
secco described in the preceding paragraph was practised
before and during the thirteenth century as the precursor
of buon' fresco, and is briefly mentioned in Theophilus
('Schedula,' Book I., chapter xv.).
The protection afforded to the pigments by the binding
material in fresco-painting is not generally very efficient.
In the case of a dry wall, free from soluble saline matter,
and exposed to a pure atmosphere, it may remain good
for centuries. But in air contaminated with the products
of the combustion of coal and gas, and with tarry and
sooty impurities, a fresco picture soon perishes. The
binding carbonate of Ume is converted into the sulphate,
breaking up the paint, and becoming itself disintegrated
in the process of change. Through the same cause, and
through the production of sulphate of magnesia from the
carbonate of magnesia in the plaster, even the layer of
paint itself may scale off, while the lodgment of dirt and
soot upon the surface obscures such colours as still remain
in their place. And fresco-paintings often show scaling-
3o6 FRESCO-PAINTING
off, by reason of the interposition of a film of carbonate
of lime between the coats of paint — a. film formed during
the completion of the picture.
True fresco did not come into use in mediaeval times
until the close of the fourteenth century. About the year
1390, Pietro d'Orvieto painted some subjects from Genesis
in the Campo Santo at Pisa. In 1503, Pinturicchio, at
Siena, began some works in fresco, which he finished in
tempera with lakes and other pigments injured by lime.
This mixed method was much used in Italy to a late
period, as it enabled a greater richness of effect to be
attained. For the palette of the painter in true fresco is
severely restricted in certain directions, very few colours
of organic origin withstanding the decomposing action of
lime. It is a good plan to test each pigment intended to
be employed in this method : The pure pigment is thinly
painted over a slab of plaster-of-Paris, and then half of it
is to be moistened with lime- or baryta-water. No change
of hue, only a lightening of the tone, should be observed,
after drying, in the treated portion. Prussian blue may
be named amongst the pigments most quickly and seriously
altered by lime ; it becomes a mere stain of rust.
Although it might have been expected that the earthy
pigments, terre verte, yellow ochre, and raw sienna, would
prove peculiarly suitable for use in fresco-painting, the
examination of works executed in this method, during
the last half-century in England, does not confirm this
expectation. Indeed, it is found that the most friable
portions of such frescoes are precisely those in which
these pigments have been freely employed. This remark
applies particularly to terre verte, which is found to have
become swollen and easily detachable.
As lime in the caustic state acts strongly upon wood.
FRESCO-PA INT IN G 307
it is necessary to employ palettes of zinc or glazed
earthenware ; bone or ivory palette-knives are preferable
to those of steel.
Asiatic Fresco. — The remarkably successful explorations
of Sir Aurel Stein among the buried sites of Chinese
Turkestan have brought to light numerous examples of
a peculiar variety of fresco-painting. In the brief ' Guide
to the Stein Exhibition of 1914,' in the British Museum,
the method is vaguely described thus : * A preparation of
lime is spread over a foundation of mud and chopped
straw, and the pigments applied to the surface while it
is wet' In reality the process adopted in these works,
dating from the third to the tenth century of our era, may
be more exactly described in the following words : On
a backing of the ordinary local loess mixed with the
chopped stems and leaves of the common reed, there
was spread a thin flat coating of impure burnt gypsum
made into a cream with water. Pigments such as an
iron red, malachite, a charcoal grey and an ochre, some-
times mixed with the cream of burnt gypsum, were then
painted on while the surface was still moist. On drying
the colours became fixed, not by carbonation, as in true
fresco work, but simply by loss of the solvent water
present and the crystallization of its content of gypsum.
As I made numerous analyses for Sir Aurel Stein of
painted plaster, from sites at Kadalik, Miran, and Mingoi,
I can speak with confidence of the essential distinction
between Asiatic and European fresco : the former is
essentially a plaster-of-Paris method.
3. In stereochromy, or water-glass painting, a process
introduced more than sixty years ago, the fixative em-
ployed is an alkaline silicate dissolved in water. From
time to time different experimenters have improved the
3o8 STEREOCHROMY
painting-grounds, the preparation of the pigments, and
the mode of applying the fixing liquid ; but the main
chemical actions involved in this method of painting are
identical in all the modifications which have been intro-
duced. The constituents and preparation of painting-
grounds adapted for this process have been discussed in
Chapter II. The pigments should be treated, as recom-
mended by Kuhlmann, with some of the fixing liquid,
and then reground ; in some cases they require the previous
addition of oxide of zinc, powdered marble, powdered glass,
carbonate of baryta, soluble silica, hydrate of alumina, etc.,
in order that their natural inaptitude for equal fixation by
the alkaline silicate should be remedied. Opinions differ
as to the desirability of treating the painting-ground with
some of the water-glass solution before laying on the
colours ; but it is essential that if a solution of this silicate
be used at this stage, it should be very dilute. The finished
painting is sprayed with a warm dilute solution of potash
water-glass or potash-soda water-glass, to which has been
added liquor ammoniae. The surface is shortly afterwards
washed repeatedly with hot distilled water ; and, if neces-
sary, the application of the water-glass solution, and the
subsequent washing, are repeated. The final result of
these operations is to bind the particles of pigment to
one another, and to the ground, by means of an insoluble
double silicate. This silicate, formed partly out of some
of the constituents of the ground, of the pigments, and of
the water-glass, mainly consists of silica, lime, and potash ;
it often contains zinc, magnesia, and alumina. The soluble
salts removed by washing the painting with water are the
carbonates of potash and ammonia ; when, however, soda
is present in the water-glass, carbonate of soda has been
formed, and is removed at the same time. The pigments
OIL-PA INTING
309
employed in stereochromy are more limited in number
even than those available in fresco-painting, and consist
chiefly of natural oxides and earths, the artificial oxides
and hydrates of chromium and iron, cobalt green, ultra-
marine, cobalt blue, and ivory-black.
4. Oil-Painting and Spirit-Fresco. — The essential charac-
teristic of these methods is to be found in the use of a
binding material which is in itself insoluble in water.
The painting-ground employed should be dry, and free
from alkali and from soluble salts. If it be primed
canvas or panel, it is a good plan to cleanse it with oxgall
and water, or with a very weak solution of carbonate of
ammonia, before commencing work. A discoloured lead-
priming should be restored to its original brightness by
laying a sheet of white blotting-paper upon it, and then
just saturating this paper with a solution of peroxide of
hydrogen. The moist surface is now exposed to a
moderate degree of heat — as by holding it in front of a
fire — which greatly quickens the activity of the peroxide.
When the paper has become dry, it may be removed, and
the bleaching of the tarnished ground will be found to
have been effected, the brown sulphide of lead having
been oxidized into the white sulphate. In order to learn
whether a plaster-ground or a wall is sufficiently dry to
be safely painted upon in oil or spirit-fresco, the gelatin-
test may be employed, A small oblong piece of coloured
sheet-gelatin is held firmly and closely against the plaster
or wall, by means of a stick applied at the centre. If
hygroscopic equilibrium have been established between
the wall and the air, the gelatin will remain flat ; if the
wall be moister than the air, the sheet will curl outwards,
the inner surface becoming highly convex. Slate and
several other suitable painting-grounds may be dried and
310 OIL-PAINTING
further prepared for work in oil or spirit-fresco by heating
them gradually in a water-oven up to the temperature of
boiling water, and then rubbing them with a piece of
hard paraffin-wax. The slate is again heated in the water-
oven, withdrawn, and then at once rubbed with a dry,
warm cloth, so as to remove all excess of paraffin wax.
Other methods of treating stone, etc., for the reception of
oil-colours have been previously given. A very con-
venient means of neutralizing the residual alkalinity of a
lime-plaster ground intended for oil or spirit-fresco painting
is afforded by linoleic acid.* This liquid fatty acid is an
article of commerce, moderate in price, and easily obtain-
able. A wide-mouth tin of it is placed in a vessel of
boiling water ; when the linoleic acid is hot, it is paid on
to the surface of the plaster with a wide brush, any excess
being removed by wiping the ground with a cloth. Solid
stearic acid may be melted and used in the same way,
but its effect is inferior.
The vehicles employed in these methods of painting are
not miscible with water— are, in fact, hydro fuge materials
repellent of moisture. If an absorbent ground or other
porous material be soaked with water, and then covered
with oil, as the water evaporates the oil penetrates, and
at last completely takes its place. But, on the other
hand, the reverse process cannot be carried out, since the
water outside will not displace the oil inside. These
vehicles are either oils or else solid substances in solution
— solids which, though insoluble in water, may be dis-
solved with more or less ease in one or other of a long
series of liquid solvents (Chapters V., VI., XI., and XII.).
The changes experienced by these vehicles and their
* By linoleic acid is here meant the mixture of fatty acids
obtainable from raw linseed oil.
OIL-PAINTING 311
constituents during the painting process may be thus
summarized :
(a) The oils used absorb oxygen from the air, increasing
in weight thereby to the extent of 10 or 11 per cent. —
such increase in weight being accompanied by a consider-
able increase in bulk. This latter change is clearly shown
when a layer of a drying oil, spread upcn glass, is allowed
to dry ; it then becomes rippled or wrinkled from ex-
pansion ; such expansion, owing to the viscosity of the
oil, takes place mainly in a direction perpendicular to
that of the surface of the glass.
(h) The above-described absorption of oxygen by the oil
employed in painting results in the formation of a sub-
stance or mixture of substances called linoxme. Now this
product is not only solid instead of liquid, but it is almost
insoluble in the usual solvents of oils unlike the oil from
which it has been formed. But there are circumstances,
not yet accurately defined, in which linoxine itself occa-
sionally suffers a peculiar change, finally becoming brown
in colour, tacky in consistence, and soluble even in spirits
of wine. This degradation of linoxine is, however, of very
rare occurrence in the ordinary practice of oil-painting.
A singular circumstance connected with the transformation
of ' linolein ' into ' linoxine ' has been noticed ; this change
is accompanied by the formation of hydrogen peroxide, a
compound which is also produced during the oxidation of
the terpenes. The continuous production of the peroxide
may be recognised on the surface of an oil-painting long
after it has been completed by the blue colour which it
develops in starch-paste containing potassium iodide.
(c) The resins present in varnishes and media contract
for some time after the major part of their volatile solvent
has escaped by evaporation, and thus leave a residue which
312 OIL-PAINTING
becomes fissured. In a properly-proportioned medium
this contraction should be balanced, or rather more than
balanced, by the expansion of the oil present. Hence the
desirability of associating a varnish (or a resin dissolved
in a volatile solvent) with a drying oil, in this method of
painting.
(d) Waxes and solid paraffins, when once deposited from
a solution by the escape of the solvent, neither expand
nor contract by desiccation or oxidation, but only through
changes of temperature.
(e) Most of the liquid solvents simply evaporate, leaving
no fixed residue due to their previous presence. But spirit
of turpentine and oil of spike generally behave differently.
Some kinds of spirit of turpentine differ from the majority
in this particular, but the remainder suffer two simul-
taneous changes. A portion evaporates ; another portion
absorbs oxygen from the air, becoming converted into a
sticky, yellow, and resinous substance, which remains
behind. The resin thus formed is a very objectionable
constituent in the structure of a picture, and its production
should be avoided either by employing a variety of tur-
pentine not subject to easy resinification, or by using a
freshly-distilled turpentine which has been secluded from
the air, and in which a few lumps of freshly-burnt lime
have been placed, to remove water and such resinous
matters as may be produced.
An important precaution to be observed in the 'conduct '
of a painting during its progress is based upon the two
actions just referred to, namely, the oxidation of the oil
during its hardening, and the escape of volatile solvents.
The latter action takes place more easily than the former,
and so if a picture is to be carried on rapidly to comple-
tion, the earlier and lower paintings should contain less
OIL-PAINTING 313
oil than those nearer the surface, into which more oil and
less resin (copal or amber), dissolved in some volatile
solvent, should be introduced. If the reverse order be
followed, the highly oleaginous layers below, having had
no sufficient opportunity for oxidizing, drying, and harden-
ing, v-/ill be rent by the strong and quickly-drying resinous
layers above them.
The harder resins, paraffin-wax, wax, and oil, possess
in varying degrees the power of ' locking-up ' the pigments
with which they are mingled, in such a way that these
become much less liable to act upon one another, and to
suffer injury from external agencies. In a measure they
repel and exclude moisture and oxygen — two of the chief
agents of chemical change. But the value of these
' locking-up ' materials has been exaggerated : they often
prove quite ineffectual in preventing the oxidation or
other change suffered by non-permanent pigments and the
inter-action of pigments. For instance, the oil which
surrounds each particle of cadmium yellow and emerald
green, in a mixture of these two oil-paints, is not capable
of preventing the formation of the black sulphide of
copper. And Dr. A. P. Laurie has found that when a
layer of linseed-oil is interposed between these oil-colours
separately spread, it is the emerald green which appears
to travel towards the cadmium yellow — perhaps owing to
its solubility in the medium. In consequence, the pro-
duction of spots of black sulphide of copper occurs chiefly,
if not entirely, on that side of the oil-layer which is in
contact with the cadmium yellow. To Dr. Laurie we are
also indebted for a very ingenious method of comparing
the locking-up function of various oils and resins. Dr.
Laurie prepared some anhydrous sulphate of copper which
is white, but acquires a blue colour when exposed to
314 OIL-PAINTING
moisture. He ground this white sulphate with various
media, painted glass slides with the mixtures, dried them
in a desiccator, and then exposed them to moist air. A
solution of amber in turpentine proved superior, in its
power of resisting the access of moisture, to boiled linseed-
oil, oil-copal varnish, amber dissolved in oil, resin or
mastic dissolved in turpentine. Another set of trials, in
which the test substance was ground in linseed-oil, allowed
to harden in a desiccator and then coated with different
varnishes, indicated a temporary superiority on the part
of mastic in turpentine, and of oil-copal varnish over
amber or copal in turpentine. The inferiority of the latter
solutions may be due to the rupture in continuity of the
resinous films which they leave on evaporation. (See
Journal of Chemical Industry, June, 1890.) But it must
not be forgotten that many an old oil-picture furnishes
distinct evidence of the value of resinous matters (such
as Strasburg and Venice turpentine) in locking up such
changeable and destructive pigments as verdigris and
orpiment. The slow and laborious execution of such
paintings constituted an important element in the success
achieved, for each layer dried and hardened before the
next was applied.
It should be noted that different oil-paints contain very
different percentages of oil. This fact should be taken
into account, so far as possible, in adjusting the amount
of resinous matter to be introduced during the course of
work upon an oil-picture. A table giving approximately
the quantities of oil required in grinding 100 parts of
various dry pigments as oil-paints will be found on
page 66. Further information concerning such pigments
is given in Chapters XIII. to XIX.
In completing an oil-picture, the three operations of
OIL-PAINTING 315
' glazing,' ' oiling out,' and * varnishing ' remain to be con-
sidered. As to glazing and oiling out, it should be stated
that drying oil, with a little copal or amber varnish, should
alone be employed — mastic varnish should never be added
to the oil. Of course oil-paints are used in admixture
v^ith oil and copal for glazing purposes. If mastic be
introduced, a risk is incurred of its partial removal during
any cleaning operation to which the picture may be after-
wards subjected. The question of the kind of varnish to be
finally applied to an oil-picture has been much discussed.
Our choice lies between a strong irremovable varnish, and
a weak one capable of being abraded by friction, or of being
dissolved by the application of a suitable solvent, which
will not touch the true painting beneath. Mastic dissolved
in turpentine fulfils the latter conditions ; copal or amber
dissolved in oil and thinned with turpentine, and mixed with
a Httle oil, constitutes a strong, hard, irremovable protec-
tion to the surface, and becomes a part of the picture itself.
Under no circumstances should any varnish be applied to
the painting until the latter has become thoroughly hard
and dry ; the danger of tearing the layers of paint by
such application will then have been reduced to a mini-
mum. A further advantage of delay in varnishing a picture
accrues through the increasing insolubility with age of
the oxidized oil present therein, the pigments associated
therewith becoming less liable to removal by any treat-
ment to which the work may afterwards be submitted.
The chemistry of Gambier-Parry's spirit-fresco method,
and of the process in which paraffin- wax and copal varnish
are employed as the vehicle, is essentially the same as that
of oil painting. The wax or paraffin-wax is introduced
merely to secure a matt surface. Pictures executed in
these methods are, of course, never varnished. The
316 SPIRIT-FRESCO PAINTING
method of spirit-fresco was devised by the late Mr.
Gambier- Parry with the object of obtaining such effects
in mural paintings as are realized in true fresco, but with
greater ease in working, and greater permanence under
adverse atmospheric conditions. He desired to exclude
linseed or other fixed drying oils completely from the
medium and other materials employed. With this end
in view, he directed that the pigments used should be
ground up, not with oil, but with the medium itself. He
was apparently unaware that the copal varnish, which
enters largely into the composition of his vehicle, contains
a greater proportion of oil than of any other ingredient
(see Chapter XII.). So, after all, the medium used in
spirit-fresco differs from that generally employed in oil-
painting rather in the proportions than in the nature of its
ingredients. Thus in working with it we shall find that
its binding character is obtained as a result of the same
two changes which cause the fixing and solidification of an
oil painting, namely, the oxidation of the oil, and the
desiccation of the resin. The wax present suffers no
chemical alteration at first, but merely solidifies, although
after the lapse of years it is liable to produce a kind
of exudation or bloom ; indeed, in the course of years the
wax may wholly disappear. It should be added that the
painting-ground for this method of working is first
prepared with the medium diluted with oil of turpentine
(see Chapter II.).
The method of painting with the paraffin-copal medium
involves the same chemical and physical changes as
those which occur in the use of the spirit-fresco vehicle,
and is carried out in the same manner. Colours stiffly
ground in oil may be used or in a mixture of the medium
with oil : dry colours ground in the medium generally
WATER-COLOUR PAINTING 31?
are to be preferred. The medium may be diluted to any
desired consistency with spirit of turpentine or with oil
of spike, but no dilution further than that required to
secure perfect freedom in the manipulation and use of the
paints is desirable, while it is important to remember that
the use of abundance of medium is necessary to bind
the particles of pigments firmly together. Artists have
sometimes found that a picture painted in spirit-fresco
will cede colour to a cloth used in rubbing its surface.
This result is due either to excessive use of a diluent
in working with this medium, or to a deficiency of oil
in the copal-varnish used. I have never known a friable
surface to be formed where the colours employed had
been ground in oil instead of in the medium, or where
a little extra oil had been added to the latter.
5. Water -coloicr Painting. — The usual binding material
in this method is gum ; glycerin and honey are also
employed to some extent. Raw honey should never be
used, but only one of the sugars it contains, known to
chemists as Icevulose (Chapter VIII.). Great care must
be taken not to introduce any unnecessary excess of
either glycerin or laevulose, as these materials attract
moisture from the air, and we know that moisture
is one of the most potent agents in causing injury to
works in water-colour. Glycerin and laevulose are,
however, useful, when employed in moderation, for pre-
serving the pigments in working condition, and in counter-
acting the tendency of gum to crack. The media used
in water-colour painting, consisting wholly of aqueous
solutions, afford very slight protection to the pigments
used. In the presence of the moisture of the ground
(paper often contains naturally 10 per cent, of water)
and of the air, water-colour pigments have abundant
3i8 WATER-COLOUR PAINTING
opportunities, not only of acting upon one another^
wherever from their chemical constitution such action
is possible, but also of being acted upon by external
agents. Thus it comes to pass that several pigments
(vermilion, for instance, and emerald green) useful in
oil-painting cannot be safely used as water-colours.
Again, there are a few pigments (such as strontia yellow)
which are soluble in water, and which consequently may
gradually sink into the paper, and so partially disappear
from the surface.
Assuming the paper-ground to be of linen-pulp, and
free from ' filling,' from bleaching substances, from anti-
chlors, and from fragments of iron, it will still contain
about 5 per cent, of size. When in preparation for
painting it is moistened with water, this size swells, and
on the subsequent application of washes of pigments,
enters partially into mechanical union with them, so
that the various coloured materials applied to the sur-
face become associated with the size rather than with
the paper-fibres. One paint, Indian ink, itself contains
size, and for this reason when washes of it are laid upon
paper previously damped, their incorporation with the
size of the latter is so intimate that their removal is
impracticable. The size in a water-colour drawing be-
comes in time partly coagulated and insoluble ; the gum
merely dries. Instances are known where the size has
in some degree ultimately perished.
6. Pastel, Charcoal, Plumbago, Silver-point. — The com-
mon characteristic of all the processes which form our
sixth group is the absence of any vehicle or binding
material. The usual ground on which drawings in
the above-named substances are executed is paper
(Chapter I.); but as the hold of coloured chalks and
PASTEL-PAINTING 3i9
of charcoal is very precarious, the paper is generally
mounted on some comparatively rigid backing, such as
millboard, cardboard, copper, or panel. If a chalk or
charcoal drawing be carried out on paper which has first
received a wash of gum-water or of dextrin-solution, it
is easy to effect a partial fixation of the powdery pigment
by subsequently steaming the finished work, although it
is usual to employ a fixing solution in the form of very
fine spray to the finished drawing. For pastel work a
specially prepared paper is now generally employed.
This has a surface of finely-powdered pumice, which
affords an efficient tooth, and helps in securing the
coloured chalks or clays. This result is further aided
by the plan of working in and mingling the pigments by
means of rubbing with the fingers and the palm of the
artist's hand. Pastel-paper is often made of inferior
pulp, and lacks strength. It should be less sized than
paper intended for water-colours. Pastel colours are
generally made with a basis of purified chalk or pipe-clay
mingled with the usual pigments in powder, a slight
degree of cohesion being secured by makmg up the
crayons with starch -paste or gum-tragacanth.
H For fixing pastel-drawings it is convenient to use
the following medium: Pound 15 grams dry casein and
3 grams of borax together, and then shake the powder
with constant stirring into 100 cubic centimetres of dis-
tilled water. After some hours a syrupy mass will have
been formed. Dilute this with more water so as to
make the liquid up to 750 cubic centimetres; then add
250 cubic centimetres of spirits of wine. After a time
a white precipitate may form ; pour off the somewhat
opalescent liquid from this sediment. This fixative is to
be sprayed on to the face of the pastel, care being taken
320 ■ PASTEL.PAINTING
to prevent the liquid from gathering in actual drops upon
any part of the drawing. When the surface looks moist
and shiny, it shows that it has been sufficiently dosed
with the fixative. The more completely the ground is
protected by the colour laid on, the less risk there is of
the fixation affecting the appearance of the picture. If,
however, the effect of the work has been obtained by a
mere whiff oi the powdery pigment, it is wiser to omit
the fixing procedure, for the delicacy of such very fine
layers of colour would thus be impaired. In any case,
the artist who is concerned for the permanence of his
work will always try to obtain his effects by building up
as solid a layer of colour as possible.
U Pastels, as already mentioned, containing no binding
material, or next to none, drawings made with them are
exempt from the drawbacks inseparable from the use of
vehicles. Consequently there is no fear of the surface
cracking, darkening, blooming, becoming brown, or other-
wise altering. When we further consider that pastel-
drawings, unlike water-colours, depend for their effect on
the presence of a fairly solid layer of pigment, and that
many colours which are unstable when employed in
other methods of painting, have proved to be durable in
pastel, we are bound to admit that this beautiful technique
is not only simple in method but expressive in the effects
which it commands ; but is only capable of producing
drawings which last better than most others, provided
they are protected by glass, and are not exposed to
damp.
H Since the pastel crayons of the shops bear usually no
indication of the pigments employed to colour them, and
frequently contain unstable coal-tar dyes, special care
must be taken to test their permanence when exposed to
PASTEL-PAINTING 321
light. To do this it will suffice to expose to direct sun-
shine in bright weather one half of a strip on which clear
tones of the set of coloured pastels which we wish to use
have been spread and fixed. It is possible, if the sun-
shine be strong, to detect the more alterable pastels after
a few days' exposure. For serious work the artist should
use only such pastels as have stood the test. It is
fortunate that, owing to the absence of any medium,
chemical interaction between pastel -pigments when
mixed together in the process of painting is virtually
non-existent.
Details concerning the making at home of pastel-
crayons will be found in W. Ostwald's * Letters to a
Painter,' English edition, pp. 22-27. Here we need add
only the following memoranda : Excellent pastel-grounds
may be prepared by laying, on Bristol board or stout
drawing paper, a thin and even coat of powdered pumice
mixed with liquefied starch (see p. 95). Or the same
coat may be spread upon a surface of a fine fabric, such
as thin calico, linen, or silk, previously secured to the
board or paper by means of starch-paste.
In plumbago (lead-pencil) and silver-point work, the
mechanical adhesion of the coloured particles is naturally
less imperfect than in pastel, a portion of the plumbago
or silver becoming, in fact, incorporated with the fibres
of the paper-ground. This is particularly the case with
silver-point, in which method the ground receives a
particular preliminary preparation. One of the best
materials for this purpose is Chinese white (oxide of
zinc). An even wash of this pigment in the form of
* moist ' water-colour is first spread over the paper. As
a silver-point drawing is often heightened with touches
of Chinese white, it is desirable to bring these into promi-
322 SILVER-POINT DRAWING
nence by tinting the ground. For this purpose a small
quantity of some permanent pigment is mixed with the
wash of Chinese white. Yellow ochre, raw umber, green
oxide of chromium, Mars violet, ultramarine with a little
ivory-black, may be used. The ' tooth ' of surface which
increases the attrition of the silver-point is, however,
furnished by the presence of the Chinese white. It
should be added that the silver used should be free from
any alloy of copper, which hardens the metal, but may
advantageously contain a few per cents, of metallic lead ;
an alloy of 2 parts of lead with i part of tin was some-
times used instead of silver. The silver in silver-point
drawings is liable to become brown from the sulphur
compounds in impure air. The blackening of the high
lights in old silver-point drawings is due to the tarnishing
of the lead white employed ; it may be got rid of by
keeping the drawing for some time in an atmosphere of
moist ozone, or by a careful treatment with a solution of
hydrogen peroxide in ether.
A pointed pencil of pure gold is occasionally used
instead of one of silver or graphite. It produces on a
prepared paper surface the same grey line, and is theo-
retically a perfect material for drawing purposes. But,
strange to relate, gold-point drawings have been observed
to suffer change, becoming nearly invisible, not by reason
of any chemical action on the metal, but in consequence
of a rearrangement of the metallic particles whereby
their grey hue disappears and the original yellow lustre
of the gold is resumed. Drawings in platinum-point are
not susceptible of this change. The platinum employed
must be pure, otherwise this metal is too hard for com-
fortable manipulation.
Pastel or coloured chalk drawings frequently show a
PA ST EL -PA INTING
323
higher degree of preservation, so far as certain hues are
concerned, than contemporary works executed in oil. One
can easily account for the pure and fresh air of old pastel
drawings, knowing that they have been carefully mounted
and framed, and that there has been no oil or resin to
yellow and darken the pigments. But how can the remark-
able state of preservation in which the ' carnations ' are
found in so many examples be explained ? Has the
intimate commixture of chalk or of clay with crimson lake
preserved the latter from the destructive action brought
about by light ? If there have been such a preservative
action, has it been physical rather than chemical ?
Answers to such questions must be reserved until the
chemistry of coloured pastels has been thoroughly studied.
It should, however, be recollected that the white basis of
coloured pastels is not always the same. In the eighteenth
century it seems to have been invariably purified chalk,
that is, 'whitening' or 'whiting,' which is essentially
calcium carbonate. But, on examining lately a well-known
make of French pastels, a considerable percentage of
calcium sulphate was recognised, in addition to chalk.
Further experiments seem to show that the colouring
matter used is first ground up with a mixture of chalk and
plaster-of-Paris, and that, in consequence, the subsequent
addition of water causes the whole to set into a mass of
just sufficient tenacity to hold together, though very soft
and fragile. In this way the use of starch-water as a
binding material is obviated. In other pastels pipe-clay
or china-clay has been employed as the basis for the
colouring matter.
Paintings and drawings executed in fresco, in tempera,
and in water-colours, may be protected from the hostile
attacks of impure air and moisture by applying to the
324 PASTEL.PAINTING
finished work a coating of pure hard paraffin-wax. If
such an after-treatment is contemplated in the case of a
work executed in water-colour the amount of vehicle em-
ployed (gum, etc.) should be reduced to the necessary
minimum. The mode of applying the paraffin-wax is
described further on at the end of Chapter XXV.,
pages 356-357-
CHAPTER XXIV
THE STUDY OF OLD PAINTINGS AND DRAWINGS
The study of old pictures, with the view of discovering
the causes of the physical and chemical changes which
have taken place in them, is fraught with interest. The
material on which they are executed, the medium em-
ployed, the pigments which can be identified, and the
varnish which has been applied to the surface, all these
matters demand attention. The dates of the various
works examined, the countries in which they have been
produced, the conditions under which they have been
preserved, and the treatment to which they have been
subjected, constitute elements in the investigation which,
whenever possible, should be kept in view. But the
adequate treatment of this extensive subject requires
not a brief chapter, but a whole volume. And then our
materials, though in some directions most abundant, are
in great measure inaccessible. We must confine our
attention to such specimens as are shown in our public
galleries. Even then we find ourselves hampered by the
impossibility of making the thorough investigation which
is desirable, and by the too frequent absence of certain
important data. In the present chapter we limit ourselves
to some general remarks, and to a few brief observations
upon a certain number of pictures in the National
32s
326 CHANGES IN PICTURES
Gallery, the National Portrait Gallery, and the Victoria
and Albert Museum ; and our selection will be confined
to paintings in oil, tempera, and water-colour, as the
available works in fresco in England are too few and too
fragmentary to furnish the information for which we are
seeking.
It will hardly be necessary, with respect to changes
in painting-grounds, to do more than refer the reader to
what has been already said on this subject in Part I. of
the present work. The causes of the decay of panels and
of the convexity which their painted surface shows in so
many cases have been already discussed. The disruption
of the ground and of the superposed layer of paint con-
sequent upon this convexity needs no further explanation.
The staining of the white priming which has been laid on
certain kinds of wood has been traced to dark-coloured
exudations of soluble organic matters. The grain of
some kinds of wood, notably of oak in pictures of the
Dutch and Flemish schools, often becomes painfully con-
spicuous in course of time, and gives to the surface-cracks
a peculiar character. The microscopic structure of
certain woods and the peculiar distribution of their histo-
logical constituents serve to explain these appearances.
The causes of the decay and cracking of gesso-grounds
in which size has been used, and the injurious mechanical
and chemical alterations which paper and primed canvas
may exhibit, have been already touched upon. The
other conspicuous changes which may be observed in
old pictures are connected with the medium, the pig-
ments, or the varnish. All these matters have been
referred to in Parts II. and III., yet there are three points
on which further discussion may not be out of place. I
refer to the number and character of the pigments used
OLD PAINTINGS 327
in early works, to the manipulation of the paint, and to
the employment of white lead. Now, the pigments to
which the earlier painters were restricted were not only
few in number, but were mainly of mineral origin. At
the first glance one sees that the Italian artists of the
thirteenth century, and of the first half of the fourteenth,
worked almost exclusively in natural inorganic pigments,
two of which stand out in their works in startUng promi-
nence, namely, vermilion and ultramarine ; and their
pigments were nearly all opaque or semi-opaque. The
absence of any pure and brilliant yellow, opaque or
transparent, from their pictures is another noticeable
characteristic. In the works of Jan van Eyck and
Rogier van der Weyden, and in those of many of the
Italian painters of the fifteenth century, the range of
colours is more extensive. Pigments which could not be
used in tempera or size, or which were semi-opaque when
employed with these vehicles, gave great richness and
variety to their works in oil. This tendency to press
into the service of pictorial art other coloured materials
besides those of mineral origin, namely, animal and vege-
table pigments in considerable variety, became more
marked as time went on. And during the nineteenth
century the progress of synthetica,! chemistry placed at
the disposal of the picture-maker a long series of pigments
— good, bad, and indifferent, — so that the chances of
introducing dangerous and fugitive colours have been
enormously increased. It is to this increase in the
number of pigments, and to their greatly extended range
of composition, rather than to their mode of preparation,
that one should attribute in great part the frequent
deterioration of modern paintings.
But the second point to which reference has been made
328 OLD PAINTINGS
is concerned with the mode of laying on colours. The
exquisitely minute and careful manipulation of Jan van
Eyck, of Fra Giovanni Angelico, of Hans Memlinc, of
Gerard Dou, of Gerard Terborch, and of many another
old master, could not have been hurried. It was solid
but smooth ; the paints hardened gradually into one
organic whole. And we could name several oil-painters
of the eighteenth century, and even of the present day,
whose work is executed in the same safe manner, and
which, were it not for the occasional introduction of
dubious materials, would be sure to remain sound for
hundreds of years, provided, of course, that the painting-
ground be satisfactory. But this careful mode of painting
does not suit the temperament, nor is it capable of
expressing the ideas of many artists. The thick impasto
and loaded colour, the eflfective brush-work, the juicy
pencil, and the dashing haste of several painters often
prove to be elements of danger.
The third point, concerning which a few remarks seem
advisable, is connected with the use of flake-white. There
are many old oil-paintings in which the only perfectly-
preserved parts of the v/ork are those in which flake-
white has been used with considerable freedom. Here
the continuity of the layer of pigment is intact, elsewhere
there are cracks and roughnesses and scalings-ofF. To
what cause is the preservation of the high lights and of
the paler flesh-tints attributable ? The association of
hardness and cohesiveness which these parts show is
traceable to the white lead. This pigment was formerly
always prepared in such a way as to contain a consider-
able quantity of lead hydrate. The particles of this
hydrate do not lie, as it were, side by side with those
of the chief constituent (the lead carbonate), but are so
ACTION OF WHITE LEAD 329
united with the latter as to form one complex compound.
This compound acts upon the linseed or other drying oil
with which it is ground, forming a substance of great
hardness and durabihty.
This substance — that is, the entire mass of the white-
lead ground in oil which has become solid, tough, and
hard — seems to contain a small percentage of a lead-
soap, formed probably out of the free fatty acids of the
linseed-oil. But whatever the complete explanation of
this hardening action may prove to be, there can be no
doubt that we must attribute to the simultaneous presence
of oil and the hydrato-carbonate of lead the preservation
of the continuity of surfaces of the whites, and of the pale
tints into which white lead enters, in many an old picture.
No other pigment in common use is capable of solidifying
the admixed oil to anything like the extent that character-
izes white lead. Now there are modern preparations of
white lead made chiefly by precipitation or the ' wet way,'
which produces a pigment containing little or no lead
hydrate. Some writers on pigments advocate the use
of these newer products. ' Why,' say they, ' should you
carefully exclude from your pictures oils, and varnishes,
and siccatives which contain lead in solution, and then
introduce the same or a like substance in your white lead
ground in oil ?' Many years ago I tried to answer such
a question as this by means of experiment. I was actu-
ated by a desire, based on theoretical considerations, of
preventing altogether the formation of lead-soaps. I
tried comparative experiments with zinc oxide, pure lead
carbonate, and the Dutch-made lead hydrato-carbonate,
or ordinary flake-white. The two lead pigments (with
which alone we are now concerned) were washed
thoroughly with distilled water and dried before being
330 ACTION OF WHITE LEAD
ground in linseed oil. The oil-paints thus prepared were
spread in duplicate series upon glass, paper, and primed
canvas ; one set was kept in a dark box, the other was
exposed to strong light. So decided was the superiority
of the ordinary flake-white over the pure carbonate, when
both series of specimens were examined after the lapse of
various intervals of time, that I was reluctantly compelled
to abandon my recommendation of the latter. Ease in
working, solidity of body, and rapidity of drying, were
not the only points of superiority ; for the films of paint,
after having been kept a year, showed differences in
hardness and in smoothness of surface which were all in
favour of the hydrated carbonate. No discoloration was
observed in the specimens exposed to light, except in the
case of the pair upon paper ; the absorbent ground had
withdrawn some of the protecting oil, and both specimens
had equally darkened. In darkness all the specimens
had become of a somewhat greyish yellow, the discolor-
ation being about equal in all the pairs, the pair spread
on paper having, as in the previous case, become darker
than the others. The late Mr. G. W. Wigner tried a
somewhat similar series of experiments, and came to the
same conclusions. I should add that these deductions
were corroborated by the results of other trials, in which
numerous permanent coloured pigments mixed in pale
tints with these two lead whites were treated in the same
way. If, however, we feel bound to recommend the
ordinary flake-white instead of pure lead carbonate, that
recommendation does not prevent us from excluding lead-
containing oils from our pictures, seeing that we possess
perfect substitutes for them, and that there is no reason
for thus multiplying the causes of possible change.
Before commenting on some of the lessons to be drawn
ILLUMINATED MANUSCRIPTS 331
from individual pictures, it may be desirable to make a
few observations on some of the changes frequently ob-
servable in old illuminated manuscripts and choral books.
The tarnishing of lead and copper pigments laid on with-
out any protection but that of gum is very frequently
seen. The darkening of vermilion is apparently capri-
cious,* but is really explicable in part by the substitution
of red lead for vermilion, and in part by the molecular
change which the latter pigment is known to suffer, and
which has been already described. Ultramarine always
stands out absolutely intact ; sometimes it acquires extra-
ordinary prominence by reason of every other pigment
on a page having altered. The red cochineal and kermes
lakes have either gone or become paler and brownish.
Sometimes fruits painted in vermilion have been shaded
or dotted with a crimson lake, but the latter has dis-
appeared, leaving nothing but a slight gummy appearance
upon the scarlet ground. Blue flowers painted in smalt
and veined with indigo show scarcely a trace of the latter
pigment. Verdigris, which is partly soluble in water,
has run and discoloured the vellum, and at the same time
has acquired a brownish hue. Sap-green, from buck-
thorn berries, has faded greatly. Lilien-griin (of the
seventeenth century), from the flowers of Iris germanica,
has disappeared.
We now cite a few pictures, out of a large number
which have been studied for the purpose of observing the
present state of the materials which have been used in
their production. We begin with some works in the
National Gallery, Trafalgar Square.
* An initial letter in vermilion, painted in the fifteenth century,
and perfectly unchanged, became black by one year's exposure to
sunshine.
332 OLD PAINTINGS
Margaritone di Magnano (1216-1293). No. 564. In
the very limited palette of this early painter in tempera
we note that the vermilion, a yellow earth, lamp-black
and a puce colour are well preserved ; the last-named
pigment may be a form of iron oxide, and corresponds
in hue to the artificially prepared oxide called ' Mars
violet.'
Giovanni da Milano (late fourteenth century). No.
579A. The crimson on the robes of two of the three
figures which occupy these panels seems to be derived
from madder, and is well preserved.
Jan van Eyck (1390 ?-i44i). No. 186 (dated 1434).
The green robe in this famous picture shows distinct
cracks, which differ in character and are larger in size
than any others in the work. The flesh-tints are per-
fectly preserved as to texture. I suspect that verdigris
has formed a constituent of the green paint employed.
Fra Giovanni Angelico (1387-1445). No. 663. The
translucent reds and purples in this work have faded
somewhat ; the green, which appears to be malachite —
' green bice ' — has stood. On the whole this exquisite
work in tempera is remarkably well preserved.
Dierick Bouts (1410 ?-i475). No. 664. Painted on
linen which had received a very thin priming ; the pre-
servation of this work, which has never been varnished,
is remarkably good except in two particulars — the red
pigment used for the sleeves, linings of robes, etc., having
faded, and the white paint on the dress of the Virgin
having partially scaled off. From certain peculiarities
in the touch, and from the minutely wrought details of
the landscape, I conclude that the medium used could
scarcely have been the usual egg-yolk tempera, but was
rather a thin size.
OLD PAINTINGS 333
Bennozzo Gozzoli, School of (fifteenth century). No.
591. The vermihon in this tempera picture is preserved
in startling brilliancy ; the translucent reds have become
rather faded and embrowned.
Melozzo da Forli (1438- 1494). No. 755. It is prob.
able that verdigris v^as employed in painting the green
carpet in this work. If so, the cracks in this part of the
picture (more conspicuous here than in other parts) would
be due to the corrosive action of this dangerous pigment.
Tuscan School (end of fifteenth century). No. 781.
The lining of the cloak of Tobias in this picture seems to
have been painted with verdigris ; it is now very dark,
in parts nearly black, although the pigment used has
evidently been mingled with much protective resin, as its
thickness is excessive when compared with that of other
parts of the work.
Gregorio Schiavone (fi. 1470). No. 630. The madder
and vermilion in the robe of one of the figures in this
tempera picture are well preserved. This is also the
case in another work by the same artist in the author's
possession.
Gheeraert David (1460- 1523). No. 1,045. The fine
crimson glazings of madder-lake in this oil picture are in
perfect preservation. The same remark may be made
concerning another picture (No. 1,432) by the same artist.
Michelangelo Buonarroti (1475-1564). No. 790. This
unfinished tempera picture affords an instance of the
stability of vermilion mixed with red-lead (in the robe of
one of the figures), of terre verte and of madder-lake.
The last-named pigment is also to be noted in the well-
preserved hatchings and stipplings on the robes of two of
the angels in No. 809.
Ridolfo del Ghirlandaio (i 483"! 561). No. 1,143. This
334 OLD PAINTINGS
oil picture, originally painted on wood but transferred to
canvas, has been repaired and repainted in several places.
But the red glazings, apparently madder-lake, and the
green colour, seemingly verdigris on malachite, are, if
original, well preserved. The preservation of verdigris
when glazed on malachite is not unusual ; the two pig-
ments are closely related chemically, and are not likely
to react upon or injuriously affect each other.
In the National Gallery there are ten portraits in wax-
pigments from the Hawara Cemetery in the Fayum,
Egypt. A few of these portraits from this Cemetery are
on canvas, but the great majority on panels of wood.
There is a rich purple paint in several of these works, a
purple which one might perhaps be inclined to identify
with Tyrian purple from Purpura lapillus and other mol-
luscs, but which the examination of certain specimens of
ancient pigments leads one to conclude to be a madder
derivative. Anyhow, it has lasted, apparently unchanged,
for some eighteen centuries. But it must be remembered
that these remarkable paintings (Nos. 1,260-1,265, and
1,267-1,270) have been preserved in darkness almost from
the time when they were executed by Roman artists in
the period 80 to 180 a.d. The other pigments in these
paintings are yellow, red and brown ochre, charcoal
black, a blue consisting of a copper-calcium silicate, a
green from malachite, and perhaps verdigris also. An
orange-red pigment may be either red lead or realgar.
The pigments, incorporated with wax, were laid on, in a
fused condition, upon a distemper priming.
It happens that some information as to the pigments
actually employed by a Greek or Graeco- Roman artist of
the second century is furnished by six specimens found in
one of the Hawara graves by Professor W. M. Flinders
OLD PAINTINGS 335
Petrie. These pigments were : white, mainly gypsum ;
yellow ochre having, however, almost the precise hue of
true antimony yellow ; red lead ; dark red due to ferric
oxide ; pink, probably derived from madder ; and the lime-
copper silicate, known as Egyptian blue.
As to British pictures in the National Gallery, we can
afford space for a few words only. The works of Sir Joshua
Reynolds generally show the fading of the crimson lake
(from cochineal) in the flesh tints, the vermilion and
mineral yellows alone remaining. The picture of the
* Infant Samuel ' may be cited as an example of the large
and wide cracks caused by the free use of bitumen (in the
dark background). Two paintings by J. M. W. Turner
may be particularly mentioned. In No. 560, * Chichester,'
the bright lake-reds in the sky have become reduced to
brown stains — anything but luminous. In No. 534, the
* San Benedetto, looking towards Fusina,' we notice how,
in a group of small clouds near the top of the picture,
where vermilion and lake have been introduced, the
vermilion remains, but the lake is now a pale yellowish
brown.
The good condition of the great majority of the pictures
in the National Gallery of British Art at Millbank is worthy
of note. In this category may be placed the works of
Mr. G. F. Watts, R.A., and a number of other paintings
out of which I select a very few for special mention. ' The
Death of Chatterton,' by Mr. Henry Wallis (No. 1,575),
painted in 1856, was retouched subsequently, so far as
the breeches of the dead poet are concerned, the crimson
lake originally employed having practically perished.
No. 1,685, < Christ Washing St. Peter's Feet,' by Ford
Madox Brown, was completed half a century ago. It
shows, so far as one can judge, no signs of deterioration.
336 OLD PAINTINGS
Of ' The Annunciation,' by D. G. Rossetti (No. 1,210),
painted in 1849, the same observation may be made.
Anyone familiar with Lord Leighton's practice and with
his extreme care in the choice of permanent pigments,
would not expect to see any change in No. 1,574, ' ^^^
Bath of Psyche,' a work, moreover, which was finished
so recently as 1890. To the critic of pigments, paintings
of flowers afford much information, partly because they
are generally pitched in a very high key, and partly
because the living flowers themselves are generally avail-
able for comparison with their representations in paint.
Two of the pictures by George Lance (Nos. 443 and 1,184)
betray the free use made by this accomplished artist of
such fugitive pigments as carmine, crimson lake, gamboge
and yellow lake.
We may now pass on to some instructive examples pre-
served in the National Portrait Gallery :
Marc Gheeraedts (1561-1635). No. 64. In this portrait,
painted probably in 1614, while the vermilion has stood,
the translucent reds appear to have faded and changed.
The white sleeves of the dress are ornamented with small
sprigs, which are now brown, and were probably originally
painted in some vegetable yellow. The reddish pattern of
conventional foliage on the cloak now clashes with the
colour of the chair, the curtain, and the table ; the hues
of all or some of these parts must have altered.
Sir Peter Lely (1617-1680). No. 509. A well-preserved
picture in most respects, but it is probable that crimson-
lake has been used for the satin dress, which is now a
pinkish grey, and clashes with the flesh tints. This work
was probably painted about 1669.
William Hogarth (1697- 1 764). No. 289. This portrait
of Hogarth, painted in 1758 by himself, affords some in-
OLD PAINTINGS 337
formation as to the pigments he employed. He holds in
his left hand a mahogany palette 'set' with eight colours.
The first of these is white lead, and remains unchanged ;
so also is the second, vermilion ; the third is a pale warm
brown, precisely the hue of faded crimson lake ; the fourth
is now nearly black and undeterminable ; the fifth is yellow
ochre, slightly embrowned; the sixth is a pale yellow,
well preserved — much like true Naples yellow; the seventh
is a grey-blue, probably much changed ; and the eighth
and last a fair lavender blue colour. The seventh pigment
may have been indigo, and the last possibly smalt. The
cap on the artist's head has certainly faded in colour;
probably it was painted with cochineal lake.
William Hoare, R.A. (1706-1792). No. 112. This por-
trait of Alexander Pope, in coloured crayons on grey
paper, shows the blues apparently intact.
Thomas Phillips, R.A. (T770-1845). No. 269. This
portrait of Faraday, painted in 1842, shows a large number
of cracks, many of them wide. Where flake-white has
been introduced somewhat freely, as in the face and hands,
the shirt and collar, and the galvanic battery on the table,
the paint has not lost its continuity.
John Partridge (1789-1872). No 342. ' Meeting of the
Royal Fine Arts Commission.' This picture was painted
in 1846. Almost every part of it is very badly cracked
through the use of bitumen, and perhaps also of much
megilp. Even the shaded portions of the faces have not
escaped, although the high lights have been preserved
where the proportion of white lead present has been large.
Many instructive illustrations of the degrees of stability
shown by pigments are furnished by examples in the
Wallace Collection at Hertford House.
Philippe de Champaigne (1602-1674). No. 119. This
338 WA TER-COLO UR DRA WINGS
picture is remarkable not only for the perfection of its
technique, but for the extraordinary state of conservation
of all the pigments, which cover a wide range of colours,
and include a transparent amber-yellow and a rose.
Sir Joshua Reynolds (1723- 1792). No. 47. Here the
fatal asphaltum asserts itself, the background resembling
a dissected map.
J. L. E. Meissonier (1815-1891). No. 291. Generally
the works of this careful painter are well preserved, but
in this small example there are to be seen a few thin long
cracks, which seem to have arisen in consequence of the
premature application of varnish to the picture before the
oil-paint was hard.
In Sir John Soane's Museum the two fine series of well-
preserved oil-paintings by W. Hogarth (1697- 1764) will
repay careful study from the point of view now being
considered.
The remaining works to which attention is now called
are a few of the water-colour drawings shown in the
Victoria and Albert Museum.
William Daniell, R.A. (1769- 1837). In one of the
water-colours by this artist the blue and green elements
have disappeared, save just in one little bit of smalt blue
in a sailor's clothes. In another drawing the sky is now
a mere dirty laboured stain.
William Green (1761-1823). No. 685. An iron red
has here become too prominent.
Samuel Howitt (1765-1822). No. 3,019, 1876. Ap-
parently well preserved, but, on further scrutiny, it seems
that the ultramarine in the shaded parts of some of the
rocks and trees stands out more prominently than it
could have done originally.
Anne Frances Byrne (1775-1837). No. 1,358, 1874.
WATER-COLOUR DRAWINGS 339
In this fruit and flower piece the indigo in the sky has
gone, while the yellow pigments and the red lakes have
suffered greatly ; the roses are blanched, and the purple
grapes have lost their crimson element.
John James Chalon, R.A. (1778-1854). No. 570. The
' River Scene in Devonshire,' painted in 1808, shows the
shaded parts of the clouds pink, from the loss of indigo.
The hills are now too pale for the trees dotted upon them,
through the fading of sap-green and gamboge, which have
been used in painting the grass. The shadows under the
ripples of the water have greatly altered, from the change
of indigo and other pigments ; they now show dirty stains
and elaborate brushwork.
No. 54, 1887, 'At Hampstead Heath,' by the same
artist, is, on the other hand, a well-preserved drawing,
which has been more recently acquired by the museum.
The blues are particularly good, although the vegetable
yellows may perhaps have faded somewhat in spite of
the care which has been taken with the drawing.
John Cristall (1767- 1821). No. 142, 1890. This
drawing seems to have kept its hues well, and affords
a good example of the style of colouring of the period
and school to which it belongs.
Jacob Xavery (painted in 1757). ^o. 15, 1872. This
has faded woefully. The sickly peaches and spectral
grapes proclaim the evanescence of crimson lake, gam-
boge, and indigo.
Mary Moser, R.A. (1744-1819). No. 160, 1881. Very
little is left of the original colour here ; the ' roses and
other flowers ' are a complete wreck.
Francis Danby, A.R.A. (1793-1861). No. 480. The
blue in the sky unchanged, but the pigments which once
modified its hue have fled.
340 WA TER-COLO UR DRA WINGS
John Varley (i 778-1842). No. 381. Hot iron reds
show themselves in great force in clouds and elsewhere —
even in the river ; the modifying organic pigments with
which they were mixed have nearly disappeared.
George Cattermole (i 800-1 868). No. 503. The crimson
lake seems to have faded from the face in this drawing.
In No. 507, painted in 1850, we have a good illustration
of the warm brownish hues produced by the deterioration
of crimson lake.
Samuel Prout(i783-i852). Nos. 1,473, 1869, and 3,056,
1876, afford examples of the stability of true ultramarine.
The skies in both these drawings are now quite out of
harmony with the architectural features, some of the pig-
ments in which must have faded. These drawings should
be compared with others by the same artist which hang
beside them, and in which the blues of the skies, as well
as some of the pigments in other parts, have faded. Here
it may be convenient to remark upon the startling prom-
inence of the skies, and sometimes of the blue distances,
in many water-colour drawings. The first glance on
entering a room in which such works are gathered reveals
the permanence of ultramarine and cobalt blue, the latter
pigment being, of course, of comparatively recent intro-
duction. But the want of harmony in such drawings
furnishes evidence at the same time of the decay of many
other pigments — of liquorice and tobacco-juice, of yellow
lake and brown pink, of indigo and rose pink.
William Henry Hunt (1790-1864). No. 1,031, 1873.
Some of the ruddy hue from the cheeks of the boy appears
to have gone. In 1,525, 1869, some of the pink in the
apple-blossom has faded ; the primrose-flowers are greener
and less yellow than they once were, probably from a
change in lemon yellow. The l^awthorn-blossom in 1,470,
WATER-COLOUR DRAWINGS 341
1869, has lost the faint rosy blush that was once visible
in some of the flowers — a delicate hue which I can dis-
tinctly recall.
The above examples, selected almost at random, must
suffice. But I may point to a different kind of injury,
from which water-colour drawings sometimes suffer, by
citing the case of W. Delamotte's * View of Christ Church,
Oxford.' This seems to have been pasted on wood, and
to have been stained in consequence ; at least, it appears
likely that the brown spots in the sky may be traced to
the mount. A work by T. Barker (No. 134, 1878) and
the * Dieppe' of J. S. Cotman (No. 3,013, 1876) furnish
additional examples of the same kind of damage.
CHAPTER XXV
CONSERVATION OF PICTURES AND DRAWINGS
It is generally conceded that a finished oil painting is
best kept in such a position that it is exposed to daylight
of just sufficient intensity for it to be well seen, the direct
beams of the sun being excluded. In darkness, or even
in approximate darkness, the lead-whites may tarnish,
and the oil and resins darken. Even with the most
moderate illumination, however, the more fugitive pig-
ments, such as the cochineal and quercitron lakes, will
in no long time alter and fade to such a serious extent
as to destroy the ' keeping ' of the work. But there is
really no need to introduce these evanescent pigments,
for every nuance the artist can desire may be produced
with paints having a sufficient if not perfect degree of
permanence. The question of artificial lighting here
comes in. Against oil lamps, properly constructed and
managed so as to avoid the production of smoke and
soot, nothing can be urged ; the same opinion may be
given in reference to the use of electric incandescent
lamps. The introduction of electric arc lamps seems
less safe, even when the illumination they afford is reduced
to the necessary minimum, for the light they emit is richer
in those rays which, as a general rule, are peculiarly
effective in bringing about chemical changes in the less
stable pigments. But the light by which pictures are to
342
CONSERVATION OF PICTURES 343
be seen is but one of the conditions out of several which
have to be considered in their conservation, though
perhaps the most important. The mode of securing a
picture in its frame is not an altogether trivial matter.
In the majority of cases the expansion and contraction
with variations of moisture and temperature, of panel
and canvas, do not correspond accurately with the similar
changes of the frame. In consequence, too great rigidity
in the system of fixing adopted should be avoided. Duly
adjusted springs or blocks of indiarubber (not vulcanized),
secured in the rebate, afford convenient means of obtain-
ing the necessary freedom of movement, while preventing
the jar caused by accidental concussions. An equable
temperature is another important condition ; on no
account should currents of hot or of cold air impinge
directly upon the front or back of a painting. Moreover,
this is not a mere question of temperature, for such
currents of air may bring in particles of dust and other
impurities, while their hygroscopic condition is sure to
vary. This question of moisture is of some moment.
For if freshly-warmed air, which is pretty sure to be
comparatively dry air, is allowed to come in contact with
panels or canvases, it will withdraw from them some of
their necessary hygroscopic moisture, and thus cause
capricious and hurtful changes of size. Such changes,
often repeated, cannot but result in the production of
cracks and fissures in the oil paintings subjected to these
varying conditions. The hygroscopic balance between
picture and air can be maintained only by a due supply
of moisture to the warmed air before the latter comes in
contact with the painting ; the warmer the air the more
moisture must be added to it. The same reasoning
applies to the entrance of cold air, but in this case, care
344 CONSERVATION OF PICTURES
must be taken that it is sufficiently dry not to deposit
water upon the picture. For the purpose of regulating
the hygroscopic condition of the atmosphere in a picture-
gallery, the introduction of a dew-point thermometer is
advisable. And there is another contrivance by means
of which the presence of the right proportion of moisture
in the air may be recognised A strip of drawing-paper,
another of primed canvas, and another of mahogany, all
three being in a normal hygroscopic state, are to be
separately balanced by means of counterpoises. When
the air gets too dry, the strips will rise, owing to their
loss of water ; when excess of moisture is present, they
will sink. So long as the equilibrium of the beams to
the ends of which the strips are attached remains practi-
cally true, the air may be regarded as in a satisfactory
hygroscopic condition. Three pairs of ordinary apothe-
caries' scales (or spring-balances) suffice for the construc-
tion of this apparatus, which should be protected by a
glass case to which the air has free access. This ques-
tion of the due amount of moisture in the air — neither
in excess nor in defect — has scarcely received the atten-
tion it deserves. But it will be allowed on all hands
that few conditions are more trying to pictures in oil or
water-colour than those caused by currents of hot, dry
air rising directly below them during the day, succeeded
by currents of cold, moist air descending upon them
down the surfaces of the walls at night.
The covering of an oil picture with glass, whatever
objections may be urged against it from an artistic point
of view, certainly secures the protection of its surface
from the solid and liquid and, to some extent, from the
gaseous impurities in the air. But the backs of pictures,
especially of those painted on canvas, are often forgotten,
FRAMING AND MOUNTING OF PICTURES 345
yet excess of moisture and deleterious vapours and gases
often enter from behind, and seriously discolour the
painting-ground, and even the paint itself. Mention has
previously been made of methods by which this cause of
injury may be prevented by means of a double canvas,
or a layer of white lead in powder mixed with starch-
water, applied to the back of the original canvas ;
American leather cloth, or even parchment-paper, affixed
to the frame behind is nearly as effective.
A few words only are requisite as to the mounting and
framing of water-colour paintings. On no account must
the back of the paper on which a drawing is executed
come into contact with any kind of w^ood, or even with
an inferior sort of paper or mounting-cardboard. Injurious
substances in the latter may travel forwards into the
painting-ground, and affect the pigments, while wood may
cause stains. Iron brads produce rust-spots. Flour-paste
is not a sound material for mounting drawings ; far better
is an antiseptic size, which may also be used for fixing to
the back of the frame the sheet of paper which is there
placed to exclude dust. If we could secure a water-colour
drawing from dust, and yet allow of the escape of any
water set free in the form of vapour when the drawing
gets, from whatever cause, somewhat warmer than usual,
we should have effected an improvement upon the ordinary
plan of framing. In this, the moisture liberated from the
paper and mount cannot escape, but condenses upon the
glass when it cools, only to be reabsorbed by that surface
of the paper which carries the pigments, where it favours
chemical and physical changes, until the hygroscopic
equilibrium of the whole system — frame, mount, lining,
paper, etc. — is once more re-established. I have used
with advantage grey linen in lieu of brown paper at the
346 PRESERVATION OF DRAWINGS
back of frames, and, by means of a few strips of thick
drawing-paper, have established an air-communication
between the space in front of the drawing and that at
the back. Thus the ventilation of the system is arranged
for, yet dust is excluded. To hermetically seal a framed
drawing, to the entire exclusion of all moisture and all
air, is not possible. That under such conditions a greatly
increased number of pigments would prove unalterable
has been long known. We should add to these observa-
tions upon the conservation of works in water-colour that
they should certainly be kept in a rather drier atmosphere
than that recommended for oil paintings.
The plan of preserving the water-colour drawings of
Turner, devised by the late John Ruskin, may fitly be
mentioned here. It was described in a letter to the
editor of the Times (October 28, 1856). The recom-
mendation is to enclose each work in a light wooden
frame, under a glass, the interior surface of which is pre-
vented from coming in contact with the drawing by
means of a raised mount. A number of such frames
(five to fifteen) are kept together in cases, which can be
carried or wheeled to any part of the room where the
drawings are to be studied. Each frame slides vertically
into grooves in the case. Professor Ruskin's reasons
include the following : ' A large number of the drawings
are executed with body colour, the bloom of which any
friction or handling would in a short period destroy.'
This argument, it will be seen, is directed against the
keeping of such works, in their unframed state, in port-
folios. Another reason given by Mr. Ruskin is that in
the case of these drawings * their delicate tones of colour
would be destroyed by continuous exposure to the light,
or to smoke and dust.' He fortifies his position in refer-
PRESERVATION OF DRAWINGS 347
ence to such exposure in a letter to the Literary Gazette
(November 13, 1858), in which he says that 'the officers
of both the Louvre and of the British Museum refuse to
expose their best drawings or missal-pages to light, in
consequence of ascertained damage received by such
drawings as have been already exposed ; and among the
works of Turner I am prepared to name an example in
which, the frame having protected a portion, while the
rest was exposed, the covered portion is still rich and
lovely in colour, while the exposed spaces are reduced
in some parts nearly to white paper, and the colour in
general to a dull brown.' ' That water-colours are not
injured by darkness is also sufficiently proved by the
exquisite preservation of missal paintings, when the
books containing them have been but little used. Observe,
then, you have simply this question to put to the public :
" Will you have your Turner drawings to look at when
you are at leisure, in a comfortable room, under such
conditions as will preserve them to you for ever, or will
you make an amusing exhibition of them (if amusing,
which I doubt) for children and nursery-maids ; dry your
wet clothes by them and shake off the dust from your
feet upon them for a score or two of years, and then send
them to the wastepaper merchant?"' Mr. Ruskin in
another letter to the Times, which appeared on October 21,
1859, wrote thus : ' I take no share in the responsibility
of lighting the pictures either of Reynolds or Turner with
gas ; on the contrary, my experience would lead me to
apprehend serious injury to those pictures from such a
measure ; and it is with profound regret that I have
heard of its adoption.' Although considerable weight is
rightly given to the opinions of Mr. Ruskin above quoted,
it must not be forgotten that all paintings of the modern
348 LIGHTING OF PICTURE-GALLERIES
school are not to be classed with those of Turner and
Reynolds in respect to susceptibility to the injurious
action of the products of the burning of gas and of con-
tinuous exposure to moderate light. When, therefore,
Mr. Ruskin wrote (in the Daily Telegraph, July 5, 1876) :
* I do not think it necessary to repeat my former state-
ments respecting the injurious power of light on certain
pigments rapidly, and on all eventually,' I find myself
compelled to reject so sweeping an assertion. That light
injures all pigments eventually cannot for one moment
be conceded. And if we could but succeed in so modify-
ing the light that illuminates our pictures as to remove
from it certain particularly active beams, we might con-
siderably augment the list of permanent pigments.
Experiments on a small scale prove that several fluor-
escent substances, such as a solution of quinine sulphate,
while intercepting dangerous rays, do not sensibly modify
the colour of the light, and yet lessen its chemical
activity. In the first edition (published 1890) of the
present handbook, I wrote : ' Possibly a transparent
screen of this character will some day be used for our
picture-galleries.' Since then an arrangement of coloured
glass — peacock blue and yellow — has been devised by
Sir W. Abney and introduced into one of the galleries of
the Victoria and Albert Museum, with the object of pre-
venting the entrance through the skylight of a great part
of the injurious rays. Thirteen years ago I used the
following words in relation to this subject : ' It is in-
structive to note how much difference exists between
different specimens of apparently colourless glass in their
absorptive power for the so-called chemical rays. These
differences may be tested by framing a strip of paper
washed with carmine and covering it crosswise w4th the
LIGHTING OF PICTURE-GALLERIES 349
samples of glass to be valued, adding for comparison a
plate of rock-crystal. Under the last-named material the
fading is nearly as rapid as it is where the pigment is
without any cover. It may be safely affirmed that
miniatures should be protected by glass, not by rock-
crystal. Further experiments on the selection of pro-
tective glasses and the construction of transparent screens
are needed. A partial discussion of this subject will be
found in the next chapter, and to this I would refer my
readers.' In this connexion a paper by Sir William
Crookes, P.R.S., may be named. It was published in
the Phil. Trans, of the Royal Society, Vol. 213 A, and is
entitled ' The Preparation of Eye-Preserving Glass for
Spectacles.'
The question of the lighting of a gallery or room where
pictures are to be displayed has been touched upon already
at the beginning of the present chapter. We would now
add that actual skylights are not without drawbacks.
One of these, especially in the case of water-colours, is
the presence in large proportion in the light from the
zenith of those rays which act most energetically upon
pigments, A few observations as to gas cannot be ex-
cluded. Gas, before and after burning, is bad for pic-
tures. The evil effects of an occasional escape of unburnt
gas are less to be dreaded than those caused by the pro-
ducts of gaseous combustion. These products are sul-
phuric acid, sulphurous acid, carbonic acid, and the
moisture which is formed at the same time. Thence
results a hot, moist atmosphere laden with these corrosive
compounds. The water- vapour condenses into the liquid
form and dissolves a part of the acids named above ; the
drops which trickle down are very injurious to paper,
wood, canvas and pigments. In any case, all the pro-
350 AIR IN PICTURE-GALLERIES
ducts of the combustion of gas should be removed from
the room as they are formed. For even when there may-
be no visible condensation of liquid, the vapours formed
are often absorbed as such by paper, canvas, etc., and do
in that form their destructive work. An illustration of
this fact is furnished by an analysis of the leather back
of an old calf-bound volume. Owing to its absorption of
the products of the burning of gas this back had decayed
and fallen off, and was found to contain over 6 per cent,
of free sulphuric acid.
With respect to the building itself in which pictures
are to be kept, our aim should be to secure as far as pos-
sible pure air, an equable and agreeable but moderate
temperature, and freedom from dust and dirt. Solidity
of construction, a continuous damp-proof course, a certain
degree of elevation above the ground-level, and double
walls enclosing an air-space, are desirable as conducing
to uniformity of temperature, and preventing the con-
densation of moisture upon the interior of the rooms.
Due provision should be made for excluding from the
galleries themselves the dust and dirt which may be
brought in by visitors. And it cannot be too strongly
urged that the supply of fresh air should not, as it were,
accompany the visitors, but be brought in from an inde-
pendent source. The place of in-take of such supply
should not be gratings near to, or on the level of the
ground, in out-of-the-way and dirty corners, the certain
depositories of uncertain rubbish. From such sources
air laden with organic and inorganic impurities can alone
come. The question of the exclusion of fog and city-
smoke may be mentioned here. Some kind of air-filter
is useful. It is astonishing how effectively the solid and
liquid particles suspended in a yellow fog may be strained
RESTORATION OF PICTURES 351
off and intercepted by passing the air through a layer of
loosely packed carded cotton enclosed between two sheets
of perforated zinc — this air-filter of course requires
occasional renewal. Moist white lead, that is, white
lead in powder reduced to a paste by admixture with
water, will absorb the sulphuretted hydrogen as well as
the sulphuric and sulphurous acid present in town air.
And if the walls of the galleries are coated with a dis-
temper paint containing white lead, this absorption of
impurities goes on continually. For these impurities are
more readily absorbed by an unprotected and properly-
prepared distemper than by the pigments in the pictures.
To secure this result the distemper should be made, not
with size, but with starch-water, just sufficiently strong
to bind the particles together and to the wall. For
further particulars as to these and other arrangements for
the conservation of pictures, especially in public galleries,
the reader is referred to a paper on the subject in the
Portfolio for 1882, pp. 106-108.
The conservation of pictures naturally leads us on to
their restoration. Picture-restoration, like some other
kinds of restoration, often involves injury, often renewal.
It is frequently difficult, sometimes impossible, to re-
establish the pristine state of the work. The operations
involved should never be undertaken by the inexperienced
amateur. And picture-restorers themselves are too often
artists who have mistaken their profession, or who have
been imperfectly trained. Many possess no power or
appreciation of accurate draughtsmanship. Look, for
illustrations, at those crucial parts, the hands and feet,
in * restored ' pictures. Nor have they that exquisite
sense of quality in colour and of delicate hues which will
enable them to fill up properly actual gaps in a painted
352 RESTORATION OF PICTURES
surface. Then the pigments they use are too often un-
safe, and their vehicles unsatisfactory ; so both soon
alter, generally becoming darker and yellower. At the
same time, the skill of some restorers in the matter of
mechanical repairs — parquetting, transferring, relining,
etc. — cannot be too highly extolled.
Several manuals of directions for restoring pictures
have been given to the world ; they are of very unequal
value. Some of these tell you nothing, for they are
intended merely to advertise the skill of the writers.
Others advocate a treatment which may be called heroic,
giving you solvents, not only for the discoloured varnish,
but for glazings and paints. In reality, no directions can
replace experience and skill. The late Max von Petten-
kofer's method is one of the best known, but it is very
rarely applicable with safety and success. The object of
this method is the renewal of the transparency and con-
tinuity of the varnish by a process of re-solution in situ.
With this intention, the picture is exposed in a closed
shallow box to the action of the vapour evolved from
moderately strong spirits of wine. This vapour dissolves
the mastic on the surface of the picture, forming once
more a spirit-mastic varnish. This, on exposure to the
air, hardens, and leaves a shining coat of resin. But this
resin, being necessarily discoloured and sinking into the
cracks of the paint, makes them more prominent, while
there is great danger of its being unequally distributed
over the surface of the work.
When the varnish of an old picture is practically
intact, but the surface is begrimed with soot and dirt, it
should not be cleansed by the direct application of water,
much less by the use of a solution of soap ; but a loaf of
household bread, not more than a day old, should be
RESTORATION OF PICTURES 353
taken, and its crumb broken up into a tin canister fur-
nished with a lid ; it is important that no pieces of crust,
and no fragments of crumb which have become hard by
drying, should be introduced. Then the crumbs should
be shaken out, in portions at a time, from the canister on
to the varnished surface, and rolled gently thereon by
means of the fingers. By repeating this operation until
fresh crumbs no longer become discoloured, it is often
possible to improve the appearance of a picture very
greatly. In any case, it affords a useful preliminary to
the removal of the old varnish where such a further step
is imperatively required. Such removal is effected by
the mechanical process of chafing. A single tear of pure
mastic resin is ground or crushed to fine powder, and
placed upon some unimportant part of the surface of the
picture ; but the operation may be begun without the aid
of the mastic-powder. A gentle rotatory movement of
the ends of the fingers soon reduces the old varnish-layer
to powder, which is then removed by means of a soft
badger-hair brush, or other suitable means. The work
should be performed in a good light, and great care must
be taken not to injure any tender glazings belonging to
the painting itself. To ascertain whether the removal of
the varnish has been carried far enough, the parts treated
may be moistened with distilled water applied on a wad
of carded cotton. When the operation is complete, and
the surface is quite dry, a new coat of mastic-varnish
may be applied, if possible in an artificially dried atmos-
phere. Sometimes a little dragon's blood, or other warm-
coloured resin, is added to the mastic- varnish, in order to
prevent the cold and raw look which a picture which has
lost its old toned varnish frequently presents. An oil-
painting in which no megilp has been used, and which
23
354 REMOVAL OF VARNISH
has received, a year after completion, the thinnest pos-
sible layer of drying-oil containing a little copal- varnish,
and then, after the lapse of a twelvemonth, its final coat
of mastic-varnish, cannot be injured by the chafing pro-
cess just described. And, even under less favourable
conditions, it is the only method which can be recom-
mended for general adoption. But it has its risks, and is
not easily applicable in the case of pictures where the
texture of a coarse canvas, or the grain of a panel, is con-
spicuously evident on the surface. To these remarks on
the chafing process, we may add that it is sometimes ad-
visable to re- varnish a picture with fresh mastic before
commencing to remove the old ; a day or two afterwards
both layers may be removed together.
The removal of old varnish by the use of solvents is a
hazardous, though easy, operation. The liquid usually
employed for this purpose is spirits of wine, of about
60° overproof, diluted with one-fourth of its bulk of
distilled water. It is applied by means of wads of
carded cotton, the action of the solvent being arrested,
when necessary, by instantly moistening the spot with
spirit of turpentine on another wad, or, in some cases,
with linseed-oil. But when mastic megilp has been
used as the painting-medium, it also, as well as the
pigments associated with it, may be removed by treat-
ment with these solvents. And it must be remembered
that some artists introduce layers or touches of water-
colours in their oil-pictures ; these are almost certain to
be affected by spirits of wine. Sometimes further injury
to them may be arrested by the application of linseed-oil.
Whenever a solvent is used in cleaning a picture, the
cotton tufts employed should be examined carefully from
time to time, in order to see that no actual pigment has
RESTORATION OF PICTURES 355
coloured them — that they are stained by nothing but the
brown varnish. Other solvents besides those named are
sometimes used in cleaning pictures, particularly where
hard or oily varnishes have to be removed. Such sol-
vents are acetone, fusel-oil, amyl-acetate, benzene,
chloroform, and solutions of caustic alkalies. Great
risk of injury attends their employment — indeed, the
application of any kind of solvent is fraught with danger,
and should never be attempted by the inexperienced.
The usual plan of filling up actual gaps in the priming
or gesso-grounds of old pictures is by means of plaster-
of-Paris. When this has set, its surface is levelled by
gentle attrition with a cork and dry whitening, or cuttle-
fish. Great care is needed in order to prevent the
pigments surrounding the place from being abraded. I
have found asbestos-putty to be an excellent substitute
for plaster in many cases ; its surface is made smooth
and level by means of a painting palette-knife. But it
cannot be tinted with water- or tempera-colours ; in
order to make it match the hues of the neighbouring
parts of the picture oil-colours must be used. In any
necessary replacements of lost colours in old oil-paintings,
it has been recommended to use not oil-colours, but
water-colours, as these can always be distinguished from
the old work, and, if necessary, removed; this can be
done on ' stoppings ' of whitening and size, as well as on
those of plaster. Tempera-pictures should, I think, be
repaired with dry pigments mixed with egg-yolk, as in
this case, when the final varnish is applied, a general
harmony of effect is produced. If water-colours are
introduced in repairing an oil-painting, the coat of varnish
subsequently added is sure to deepen and darken them,
unless this change has been allowed for during the pro-
356 RESTORATION OF PICTURES
gress of the work, by no means an easy thing to manage.
When in any kind of repainting oil-paints are used, they
should be mixed stiffly with a very little copal-varnish
and spirit of turpentine, and should be rather lighter
and less yellow in tone than the colours they are intended
to match, since darkening and yellowing in some degree,
however slight, are sure to occur subsequently.
The cleansing and restoration of paintings executed in
fresco require special care. Additions to supply colour
which has scaled ofT are best made in tempera. When a
fresco has become grimy by exposure to the smoky air
of a city, methylated spirits of wine, applied freely on
tufts of carded cotton, removes the tarry and sooty im-
purities which a previous careful brushing of the painted
surface has failed to dislodge. Attempts to clear the
clouded portions of an injured fresco by means of dis-
tilled water or aerated distilled water are usually attended
with but slight success. The films which obscure the
surface in such cases sometimes consist of sulphate of
lime, produced from the carbonate of lime present by the
action of the sulphuric acid occurring in the atmosphere
of places where gas and coal are burnt. In getting rid
of this somewhat opaque film by means of water, portions
of the pigment are commonly removed. When a fresco
has been dusted and then cleansed with spirits of wine it
should be allowed to dry thoroughly, the lost colours
renewed in tempera, and then the whole surface coated
with a preparation of hard paraffin-wax. This prepara-
tion, which has the consistency of an ointment, is made
by melting together 4 parts of hard paraffin-wax
(melting-point above 150° F.), i part of spirit of turpen-
tine, and 15 parts of toluol. When cold, this mixture is
to be spread uniformly over the painted surface, and then
RESTORATION OF PICTURES 357
allowed to remain until its volatile constituents have
disappeared. Afterwards the paraffin -wax left on the
surface is to be melted and driven in by the local applica-
tion of a moderate heat. By this treatment the dead or
matt surface of the fresco is preserved, the obscuring
films are rendered translucent, and the picture may,
when cleansing is again required, be safely sponged with
pure water or weak spirits of wine. If any cloudiness of
the surface still persists after the application of the
paraffin-wax paste described above, the effect of treat-
ment with the Gambler -Parry medium (see p. 142),
largely diluted, may be tried.
The treatment of an injured fresco, in accordance with
the plan just outlined, was pursued in the case of Sir
Edward Poynter's fresco painted in 1872-73 on the south
side of the chancel in St. Stephen's Church, Dulwich.
That the work done upon this damaged fresco has been
successful may be learnt from the way in which
Mr. James Ward wrote in 1909 of the then state of the
painting in his book ' Fresco Painting,' on page 30. He
there says that this fresco is ' in a perfectly sound con-
dition, and is almost as fresh-looking and bright as when
first painted ; ... it shows no sign of deterioration ; on
the contrary, the surface looks, and feels to the touch,
more like terra cotta, or of the texture and firmness of
biscuit porcelain than anything else one can think of.'
Mr. Ward would have come to a very different conclu-
sion as to the permanence of fresco had he seen this
painting when I took it in hand some four years before
his approval was published !
An example of the treatment of a greatly damaged
oil-painting on a plaster ceiling may be here cited. This
work, in the Saloon of the Queen's House at Greenwich,
358 RESTORATION OF PICTURES
was painted between the years 1626 and 1635 by Orazio
Gentileschi, a Pisan artist invited over to this country
by Inigo Jones. It was reported in 1853 to be 'much
damaged ' ; and fifty years afterwards, when I first
examined it, its condition seemed well-nigh hopeless.
The plaster ground had swollen, and had broken up and
loosened the layer of oil-paint applied to it. This injury
was due to the action of atmospheric sulphuric acid upon
the calcium carbonate of the plaster. By spraying the
whole surface with Gambier-Parry's spirit fresco medium
considerably diluted, the coloured flakes which were
ready to fall were secured in position, and then the lost
portions were replaced by pigments ground in the same
medium. These operations were carried out between
the years 1907 and 1909. In cases of such serious
damage as this of Gentileschi's ceiling, further treat-
ments are necessary, as the injury to the plaster is a
CHAPTER XXVI
TRIALS OF PIGMENTS
The testing of pigments for genuineness and for purity-
has been discussed incidentally in Chapters XIII. to XIX.
of the present volume. Though genuineness and purity"^
have often an important bearing upon the question of
permanence, this last quality must be ascertained by
independent experiments of another order. The study
of old paintings often furnishes us with results of some
value, the results of unintentional testings. But for the
purpose of securing complete and wholly trustworthy
information, we must know precisely all the materials
and all the conditions of our trials. Not only must the
painting -grounds, the mediums, and the pigments, be
chemically examined, but we must be in a position to
state the character of the atmosphere in which they have
been exposed, and the nature and amount of the solar or
other radiations to which they have been subjected. In
the great majority of these trials accurate data as to
* The chromatic values of pigments— their approach in hue, etc.^
to recognised standards of excellence — are not here taken into
account. Such data may be obtained by the use of Lovibond's
Tintometer when once the chromatic elements of a pigment in
terms of the degrees of the standard glasses employed in this
instrument have been determined. But really exact determinations
of this kind require complex scientific apparatus, which cannot be
profitably used except by an expert manipulator.
359
36o TRIALS OF PIGMENTS
materials and conditions are wholly wanting ; even the
South Kensington report affords us no information as to
the composition of the pigments employed, nothing more
than their commercial names, so that we have to take on
trust their genuineness and purity. However, in this
same most important series, quite unusual, if not un-
precedented, care was taken in order to determine the
conditions, physical and chemical, under which the pig-
ments were tested. In my own experiments, carried on
between 1856 and 1879, in somewhat desultory fashion,
and extended and improved since 1880, the influence of
purity of sample, of the presence of moisture and of oxygen,
and of the nature of the * light,' has not been neglected ;
the full details of the methods adopted, and of the results
obtained, could not be appropriately introduced into an
elementary manual. Mention will be made of the chief
conclusions reached in the present chapter ; there are
also numerous references to them in Chapters XIII.
to XXII.
In many early treatises on painting we find observations
as to the degrees of stability possessed by various pig-
ments, along with suggestions as to methods of treatment
by which in certain cases greater permanency may be
attained. To some of these observations and suggestions
reference has been made in those chapters of the present
volume which deal with pigments ; many of the remainder
are now without practical importance, referring as they
do to pigments which have been rightly discarded.
It is only of recent years that trials of pigments have
been made with any approach to exactness. But in the
majority of cases no information has been secured con-
cerning the purity and genuineness of the pigments with
which the experiments have been made. I have not been
TRIALS OF PIGMENTS 361
able to find a single chemical analysis of any one of the
pigments tested. The chromatic analysis of the light they
severally reflect has, however, been recorded in the case
of the water-colour paints examined by Dr. Russell and
Sir W. Abney, who have likewise furnished some par-
ticulars as to the intensity of the solar radiations to which
the pigments were subjected.
More than a century ago Sir Joshua Reynolds tested,
in a rough way, the stability of some of the paints he
employed. Two causes detract from the value of his
results. In the first place, the information furnished con-
cerning the nature of many of the pigments he tried is too
imperfect to be of any use ; secondly, we are not ac-
quainted with the conditions under which his specimens
have been kept during the whole period since they left
his hands. Two of his trial canvases are preserved in
the Royal Academy. Although the darkening and em-
browning of the surface are general, and though the
names of the pigments employed are often undecipherable
or meaningless, yet something useful remains. From
the experiments made in 1772 we may gather the follow-
ing facts : The proper hues of several pigments remain
in a measure — orpiment, or kings' yellow in crystals ;
yellow lake, with wax and drying oil ; gamboge, with
lake and Venice turpentine ; gamboge, with (Venice)
terpentine ; prepared gamboge, with wax ; and verditer,
with varnish. On the other hand, gamboge with oil,
lake with oil, and many other pigments of organic origin,
when unmixed with wax or varnish, are names only, or,
at the most, brown discolorations.
M. J. Blockz, in his ' Peinture a I'Huile,' gives the
results of a number of experiments made by M. J. Dyck-
man. He condemns, for various reasons, not only asphalt,
362 MR. ANDREW'S EXPERIMENTS
but also terre verte, cobalt green, emerald green, raw
sienna, raw umber, ivory brown, and all burnt madders.
Cassel earth was slightly changed; brown ochre, burnt
sienna, Mars brown, ivory black, and vine black, proved
to be permanent. His lists of incompatible pigments are
somewhat unnecessarily extended, not being justified, in
all particulars, by further and more careful experiments.
The experiments of the late F. W. Andrew, formerly
of the Victoria and Albert Museum, have been carried on
since 1876, but have been confined to water-colours, both
moist and cake. His chief results will be found recorded
along with my own, in Part HI., in the paragraphs
devoted to the consideration of the several pigments. His
water-colour washes, generally spread on Whatman paper,
or Whatman board, were exposed for periods varying
from twenty-eight to eighty-two months, in a south
window, to all the sunshine available ; half of each wash
was doubled back, and so far excluded from light. In
some cases a third part of the coloured slip was exposed
to the air and light without the protection of glass. The
chief paints which were unaffected, at all events, so far
as some specimens were concerned, by the exposure, are
included in the following list : Yellow ochre, raw sienna,
deep cadmium. Mars red, light red, Indian red, oxide of
chromium, Leitch's blue, cobalt, artificial ultramarine,
Prussian blue, raw umber, burnt umber. Naples yellow
(true) became blackish in darkness, but was unaltered in
light, while orange chrome showed dark patches. Further
details must be given as to the pigments which were
affected. In the tabular statements appended a selection
from the results afforded by eleven sets of experiments
is recorded, the letters C and M prefixed to the entries
respectively denoting the employment of cake or moist
TRIALS OF WATER-COLOURS
363
colours. The numerical values representing the residual
hues are rough approximations only, but in some instances
they were determined by means of comparisons with
standard coloured liquids, contained in glass cylinders
graduated into ten equal parts. Hellige's colorimeter or
the tintometer of Mr. J. W. Lovibond, of Salisbury, may
be employed with advantage in these estimations.
CHANGES IN WATER-COLOURS
Name of
Months of
Residual
Depth Residual Hue
Pigment
Exposure
{Original
= 10) and Remarks
M.
Aureolin ...
28
10
... Verges on orange
M.
Aureolin ...
62
10
[yellow
M.
Aureolin ...
82
9
C.
Gamboge...
28
4
M.
Gamboge...
28
9
M.
Gamboge...
82
7
M.
Yellow lake
62
2
. . . Yellowish grey.
M.
Yellow lake
82
C.
Yellow madder .
28
I
M.
Yellow madder .
28
3
... Pinkish grey.
M.
Yellow madder .
62
I
C.
Indian yellow .
28
... 8,9
M.
Indian yellow
28
9
M.
Indian yellow .
60
7
M.
Indian yellow
82
6
M.
Pale cadmium .
82
I
... Palebufif.
M.
Vermilion
62
—
... Blackish.
M.
Vermilion
82
—
... Black.
M.
Carmine ...
28
... Pale grey.
M.
Crimson lake .
28
... Greenish grey.
M.
Burnt carmine .
28
...
C.
Pink madder
28
M.
Rose madder
28
M.
Rose madder
62
I
... Pinkish grey.
M.
Rose madder
82
II
M.
Madder carmine
62
2
M.
Madder carmine
82
M.
Purple madder .
28
4
... A pale wash.
364
MADDER VERSUS COCHINEAL
Name of
Months of
Residual .
Depth Residual Hue
Pigment
Exposure
{Original
= 10) and Remarks
M. Purple madder .
62
7
M. Purple madder .
82
2
C. Brown madder .
28
I
Warm grey.
M. Brown madder .
28
I
II
M. Brown madder .
82
C. Indigo
28
5
... Greenish grey.
M. Indigo
62
I
M. Raw umber
60
9
... Rather yellower
M, Vandyke brown . .
28
7
M. Vandyke brown . .
60
I
M. Bone brown
62
8
The pigments containing lead, such as the ordinary
chromes, and those having a copper basis, Hke emerald
green, had altered capriciously, losing part of their
original colour, and becoming tarnished or embrowned in
patches. Brown pink faded like the yellow lakes, but
acquired a bluish-grey residual hue.
The testing of the madder colours is so important that
I introduce here a few additional experiments selected
from my own note-books. The washes of the moist-colour
paints were, as far as possible, of the same depth of tone,
and they were all exposed together in a glazed frame to
one year's sunshine :
Nufne of
Piginent
Rose madder
Madder carmine
Madder carmine
Madder red
Purple madder
Brown madder
Residual Depth
{Original = 10)
Change of Hue, etc.
Slightly more purplish.
Almost unchanged.
Much more purplish. This sample
was from another source.
Less red, more purplish.
Duller, less red, more blue.
Less red, more yellow-brown.
In contrast to the above results with madder carmine,
the following experiment with the ordinary carmine
PROFESSOR ROOD'S EXPERIMENTS 365
(prepared from cochineal) is instructive. On a sheet of
Whatman paper, a space of 10 inches in length by 4 inches
in width was covered with a uniform wash of the moist
paint, having a depth of tint about equal to that of the
petals of the old China rose. This coloured strip was
then subjected to summer sunshine in such a way that
successive single inches of its length received the Hght
(during the same hours of similarly bright days) for periods
of 2, 4, 8, 12, 20, 26, 30, 40, and 100 hours, one single
inch at one end being, however, protected completely from
all access of light. The exposure of 100 hours sufficed
to bleach the last breadth completely, but had the rate of
fading been in a simple arithmetical progression, a much-
shorter exposure would have sufficed. In fact, the
bleaching action was far more energetic during the first
period of two hours than during the second, about 20 per
cent, of the original colour having been destroyed during
these two first hours, while during the second equal
period the loss of depth did not exceed one-tenth of this
amount. Moreover, it was noticed that the change of
hue consequent upon the first exposure was different in
kind to that which occurred subsequently.
Professor O. N. Rood's Experiments. — In his ' Modern
Chromatics,' pages 90 and 91, Professor Rood gives the
results of a few trials which he made as to the effect on
washes of water-colours laid on ordinary drawing-paper
of three and a half months' exposure to summer sunlight.
These pigments were unaffected :
Cadmium yellow, yellow ochre, Roman ochre.
Indian red, light red, Jaune de Mars.
Terre verte.
Cobalt, French blue, smalt.
Burnt umber, burnt sienna.
366
PROFESSOR HARTLEY'S EXPERIMENTS
The following pigments were all affected. The sequence
represents the amount of alteration, the list commencing
with those colours which suffered but little change :
Name
Nature
Name
Nature
of Pigvtent
of Change
oj" Pigment
of Change
1. Chrome yellow
2. Red lead - -
Slightly greenish.
13. Hooker's green -
More bluish.
Less orange.
14. Gamboge - - -
Fades, greyish.
3. Naples yellow -
Slightly greenish
15. Bistre - - - .
Fades, greyish.
brown.
16, Brown madder -
Fades.
4. Raw sienna
Fades, yellower.
17. Neutral tint - -
Fades.
5. Vermilion - ■
Darkens, brownish.
18, Vandyke brown
Fades, greyer.
6. Aureolin - - -
Fades slightly.
19. Indigo ....
Fades.
7. Indian yellow -
Fades slightly.
20. Brown pink - -
Fades greatly.
8. Antwerp blue -
Fades slightly.
21. Violet carmine -
Fades greatly,
9. Emerald green
Fades slightly.
brownish.
10. Rose madder -
Fades slightly, pur-
22. Yellow lake - -
Fades _ greatly
plish.
brownish.
II. Sepia - - • •
Fades slightly.
23. Crimson lake -
Fades out.
12. Prussian blue -
Fades somewhat.
24. Carmine • • -
Fades out.
Professor Rood adds that rose madder, brown madder,
and purple madder were all a little affected by an exposure
to sunshine for seventy hours, and that pale washes were
completely obliterated by a much shorter exposure to sun-
shine in the case of carmine, dragon's blood, yellow lake,
gall-stone, brown pink, Italian pink, and violet carmine.
W. N. Hartley's Experiments. — On September 4^
1886, the late Sir W. N. Hartley read, before the
British Association at Birmingham, a paper on *The
Fading of Water Colours.' His trials as to the effect on
pigments of a comparatively brief exposure to intermittent
sunshine in pure air may be thus summarized. Washes on
the best drawing-paper were the subject of the experi-
ments :
Gamboge. — Pale washes were completely bleached in
three days ; in a week strong washes were much lightened
in colour, and rendered dull, even three hours' exposure
producing a very visible effect.
Crimson lake. — Six hours' exposure to sunlight and air
almost bleaches pale washes, while three days or eighteen
to twenty-four hours of intermittent sunshine cause dark
PROFESSOR HARTLEY'S EXPERIMENTS 367
crimson tones to become very much lighter, the hue of
the pigment being altered.
Light red. Indian red, and vermilion were unaffected.
Olive green and brown pink were rendered lighter in
colour by six hours' exposure, the former pigment be-
coming bluish and the latter brownish in hue.
Indigo, cobalt, and artificial ultramarine were unaffected.
Brown madder became rather lighter after eight days'
or forty-eight hours' exposure.
Bistre faded with great rapidity, a light wash appearing
much paler after six hours.
Sepia. — A pale wash became colder in hue, but not very
perceptibly paler.
In a second series of experiments, sectors of paper discs,
washed with various pigments, were enclosed between
glass-plates, the edges of which were fastened with
gummed paper. Under these circumstances, crimson
lake and bistre were found to have been considerably
altered by five hours' exposure — somewhat more so,
indeed, than was the case when these pigments were
freely exposed to the air.
All the results above noted are in practical accord with
those obtained by other observers. The exposure to
intermittent sunshine ' for six hours a day during fourteen
days,' does not produce a sensible effect upon vermillion
and indigo. Had Sir W. N. Hartley extended his obser-
vations a few weeks longer, his conclusions as to these
pigments must have agreed with those which we have
given, and therefore with the unanimous verdict of all
other scientific observers. His statement that ' indigo is
permanent' (British Association Report, 1886, p. 581)
must, therefore, be modified into, ' indigo appears to have
suffered no change after fourteen days' exposure to inter-
368 PROFESSOR HARTLEY'S EXPERIMENTS
mittent sunshine.' A similar alteration is demanded with
regard to the stability of vermilion.
Sir W. N. Hartley's experiments with water-colour
washes on paper enclosed between glasses require a few
words of comment. He is clearly and rightly dissatisfied
with this method of trial. A supply of atmospheric
oxygen, and of hygroscopic moisture, amply sufficient for
large chemical alteration and oxidation of the enclosed
pigments, was certainly present. And the glasses did
accelerate the action, not because of ' the very slight tint
of the plate-glass,' but in spite of it. This acceleration
of change is mainly caused by the continued presence of
moisture in the confined space between the two glasses —
it cannot escape as it freely, and to a very great extent
does escape, when a piece of tinted paper is exposed to
sunshine in free air. I showed, indeed, in my lectures
at the Royal Academy, so long ago as 1880, that the
fading of many fugitive pigments is greatly lessened, when
not altogether prevented, by enclosing the paper washed
with them in a glass tube, the air of which is kept dry by
means of some strongly hygroscopic substance. When
both moisture and air are excluded (using a sealed
vacuum tube), the suspension of fading and alteration of
hue is still more marked and general.
It should be added here that Sir W. N. Hartley found
that cadmium yellow and Indian yellow are bleached by
peroxide of hydrogen, and changed into a muddy yellow
by sulphurous acid. This reagent bleaches artificial ultra-
marine and dulls vermilion. He attributes the partial or
complete destruction of the blue component of the hues
in certain old drawings, which have been long exposed to
air and light, to the presence of acids or acid substances in
the air, in the paper, or in the red ferruginous pigments
with which the blue colouring substances in question have
MR. W. SIMPSON'S EXPERIMENTS
369
been associated. These blue pigments could have been
nothing other than Prussian blue, indigo, or natural ultra-
marine. I have ascertained, by direct experiments on old
drawings, that the latter was but rarely employed for
mixed tints, but it is quite probable that the reds prepared
from colcothar, with which it may have been occasionally
mingled, would sometimes contain enough acid salts
(certain ferric sulphates) to destroy its colour. The pro-
ducts of the burning of gas and of coal would also be rich
enough in sulphuric acid to produce the same effect. I
am unable to endorse Sir W. N. Hartley's statement that
the best drawing-papers contain free sulphuric acid, at all
events when fresh from the mill, but they soon acquire
it when kept in an urban atmosphere.
Mr. W. Simpson's Experiments. — Some washes of
water-colour, of thirty-one different kinds, were made
upon cards by the late Mr. W. Simpson. He so cut the
cards as to divide each coloured strip in half ; one
section was preserved in darkness, the other was exposed
in an eastern aspect on the shutter of a house in London
for fifteen years, but the sun did not shine upon the
specimens after ten o'clock in the morning. As they
were not tightly framed, the cards became a good deal
discoloured by the absorption of noxious vapours and
dirt. The results were :
Na7ne
Natu7-e
Name
Nature
of Pigment
of Change
ofPigtnent
ofChange
Yellow ochre -
None.
Purple madder -
Hue altered.
Indian yellow
Faded considerably.
*Brown madder -
Loss of redness.
Lemon yellow
None perceptible.
Emerald green -
Slight.
' Newman's per
Cyanine blue
Apparently none.
manent yellow
None.
*Prussian blue -
None.
Cadmium yellow
Perhaps browner.
French blue - -
Faded very slightly.
Chrome yellow
Faded considerably.
Cobalt - . - ■ -
None.
Brown pmk -
Faded.
Ultramarine - -
None.
Vermilion - -
None.
Indigo, rather
Light red - -
None.
deep .- - - -
Very pale grey.
Indian red
None.
Burnt sienna - -
None.
Crimson lake -
- Gone.
*Vandyke brown ■
None.
Carmine - -
Gone.
*Sepia - - . -
Faded very slightly.
Madder lake -
More purplish.
*Bistre . . . -
None.
24
370 TRIALS OF PIGMENTS
It will be noted that the above results are for the most
part in agreement with those recorded by other experi-
menters ; the chief exceptions are marked with a star.
Vermilion is usually blackened, but it is possible that
the sample employed in these experiments was the less
changeable native form or cinnabar. The Vandyke
brown, too, was probably the earthy rather than the
bituminous variety ; the slightness of the change recorded
for madder brown and sepia, and the absence of any
alteration on the part of bistre, are less easy of explana-
tion. The madder pigments seem to have stood more
than usually well, but they often exhibit large differ-
ences of stability. Nor must it be forgotten, in assigning
values to the above results, that this trial of fifteen years'
exposure was not of the severest kind. Although, on the
one hand, there was the imperfect exclusion of an in-
jurious London atmosphere, on the other hand, the
energy of the solar radiation was much reduced by the
prevalent condition of the smoky air, while the inter-
mittent and capricious sunshine of the Metropolis never
fell on the trial cards after ten a.m.
The late Mr. R. H. Soden-Smith kindly placed at my
disposal a large number of specimens of old water-colour
cakes and of powder colours intended for oil-painting.
One set consists now of eleven cakes or fragments
of cakes (in their original box) bought about the year
1815 of Newman, in Soho Square. This set is peculiarly
interesting as the colours, which all bear the name of
the maker and his device, represent those used by many
of the best English water-colour painters during the first
quarter of the nineteenth century. The cakes are :
Indian yellow, raw sienna, raw umber, burnt sienna,
burnt umber, vermilion, carmine, burnt carmine, pink
SOUTH KENSINGTON REPORT 371
madder, ultramarine, indigo ; neutral tint and sepia are
missing. On comparing the hues of the first nine of
these paints and of the indigo with the hues of the
corresponding cake-colours as sold by the same house
in 1886, no appreciable differences were detected save
in the case of the raw umber. Here the pigment of
1 815 showed a more beautiful nuance than that of 1886.
On making comparative tests of the stability, under
exposure to sunshine, of the two sets of pigments, the
results were found to be practically identical. One cannot,
therefore, claim for the water-colour paints in use one
hundred years ago a degree of permanence greater than
that possessed by their representatives of to day.
By far the most important series of trials of water-
colour paints yet published is that to be found in the
report by the late Dr. Russell and Captain (now Sir W.)
Abney to the Science and Art Department (1888). The
reporters endeavoured to give precision to their ex-
periments and their conclusions by a careful comparison
of the effective radiation from different sources of light.
The first part of their report contains a very useful
discussion of the relative values of direct sunlight, light
from clouds, and from an overcast or clear sky, and light
from artificial sources. Several cognate subjects are also
discussed therein, such as the number of years of ex-
posure which pigments would require, if in the picture
galleries of South Kensington, in order that they might
suffer the same changes as those caused by three or
twenty-two months' exposure in a southern aspect out-
side the Museum. Part II. of the report contains the
results of twelve sets of experiments with various pig-
ments. In all the series the same paper (Whatman's)
was used. In the paragraph relating to this subject
372 SOUTH KENSINGTON REPORT
there is, however, one curious error, and one obscure
statement (p. 27). It is quite impossible that the
paper used — its weight per ream is not given — could
have contained so little as ' nearly i grain ' of sizing
matter per square foot ; 10 grains is a more probable
quantity. The sentences next following do not state
the condition of the papers which absorbed from a moist
atmosphere from 12*07 ^o 12-46 per cent, *of their weight
of water.' Were they dried previously, and, if so, at
what temperature ? We ought to have been told within
what limits the percentage of water in these papers varied
during the course of the trials : I have pointed out for
many years past the importance of this hygroscopic
moisture in paper in reference to the fading of pigments.
Eight tints of each pigment were applied to strips of
paper 8 inches long by 2 inches wide ; they were exposed
in tubes open at both ends, but having the upper ex-
tremity curved downwards so as to exclude wet and
dirt. Of course, exposure on a wall facing nearly south
constituted a very severe test, yet the circulation of air
In the tubes was more advantageous to the pigments than
would have been the steamy heat of a closed vessel, or
even of an ordinary paper-backed picture-frame. But on
the other hand, this arrangement allowed the free access
to the pigments of any noxious gases, such as sulphurous
and sulphuric acids, and sulphuretted hydrogen, which
might have been at any time present in the atmosphere.
The general results of this first series of trials are
gathered in the following table, the exposure in all cases
asting from May, 1886, until March, 1888. The pig-
ments are arranged in the order of instability, the most
fugitive being placed first :
SOUTH KENSINGTON REPORT
373
*Carmine.
♦Crimson lake.
''Purple madder.
*Scarlet lake.
♦Payne's grey.
♦Naples yellow.
♦Olive green.
♦Indigo.
*Brown madder.
♦Gamboge.
* Vandyke brown.
♦Brown pink.
♦Indian yellow.
Cadmium yellow.
Leitch's blue.
♦Violet carmine.
*Purple carmine.
♦Sepia.
Aureolin.
Rose madder.
Permanent blue.
Antwerp blue.
Madder lake.
Vermilion.
Emerald green.
Burnt umber.
Yellow ochre.
Chrome yellow.
Lemon yellow.
Raw sienna.
Indian red.
Venetian red.
Burnt sienna.
Terre verte.
Chromium oxide.
Prussian blue.
Cobalt.
French blue.
Ultramarine ash.
The pigments marked with an asterisk were found to
have distinctly altered either in depth or hue by a much
shorter exposure, from May to August 14, 1886.
In a second series of trials the tinted papers were
dried, and then introduced into the tubes, which had
been previously heated ; the specimens were then sealed
up hermetically ; as no moisture-absorbing material was
enclosed with the papers, traces of water must have been
present. My own much earlier results were abundantly
confirmed by those obtained in this series, for the number
of pigments which proved to be permanent under these
conditions was double that of the first series. Brown
madder and Prussian blue were, however, acted upon in
this second series. Dr. Russell and Sir W. Abney make
the remark that of the eight colours which remained
unchanged in dry air, but were acted on in ordinary air,
all, with the single exception of madder lake, are mineral
colours. But this is not correct, for the pigments named
are — madder lake, olive green, Payne's grey, sepia, Naples
yellow, cadmium yellow, emerald green, and burnt umber,
and of these the first four are either wholly or partly of
organic origin, while the seventh contains an acetate.
374 SOUTH KENSINGTON REPORT
In the next series of experiments, the pigments were
exposed in the presence of moisture-laden air. Very few
colours withstood this test — none of organic origin ; both
Prussian blue and Antwerp blue were entirely destroyed.
An atmosphere of moist hydrogen gas was employed
in the fourth series. Under these conditions carmine,
crimson lake, madder lake, brown madder, olive green,
indigo, Payne's grey, sepia, and Vandyke brown, suffered
no change.
When, as in the fifth series, both moisture and oxygen
were excluded, scarcely any even of the most fugitive
pigments were affected. Vermilion, however, as in all
the other experiments, became black. We know that
the reason for the change is physical, not chemical.
In the sixth series it was proved that the addition of
ox-gall had no beneficial effect in lessening the change of
hue and tone in fugitive pigments.
The remaining series, save the twelfth and last, were
devised in order to learn what influence upon the stability
of pigments might be exerted by admixture with Chinese
white, by exposure to the light of the electric arc, by
heat without light, by heat and light together, and by
exposure to the light transmitted through coloured
glasses. Amongst the results recorded, we may note
the decided changes in several pigments caused by ad-
mixture with Chinese white, and by heating the prepared
paper slips in sealed tubes for seven hours a day for
three weeks, all light being excluded. In a twelfth series
of trials, the pigments were exposed in a picture-frame
under glass in such conditions, and to such an amount of
light, as might be taken to represent the ordinary cir-
cumstances in which pictures are kept. The frame was
exposed from August 6, 1886, until May 6, 1888, to very
SOUTH KENSINGTON REPORT 375
bright light, but not to sunshine. Gamboge, indigo,
Naples yellow, brown pink, carmine, and Vandyke brown,
had faded in varying degrees. Some remarks on these
results will be found further on in the present chapter ;
they are of extreme importance, considering the large use
that has been made of these pigments by our water-
coiourists, and the mild treatment to which they were
subjected during the short period of twenty-one months.
For the results obtained with mixtures of pigments
under varied conditions of exposure, we must refer our
readers to the report itself. It may, however, be re-
marked that, in the great majority of cases, the changes
of tone and hue which occurred were such as might have
been predicted from the known degrees of stability of
the several constituents of the mixtures. Here, as else-
where in the report, we find frequent mention of the
strange, but long known, recovery in darkness of its colour
by Prussian blue which has been bleached by sunlight.
In the fourth appendix to this report is an instructive
list of the pigments employed by some of the most dis-
tinguished artists using water-colours. Forty-six painters
replied to the invitation of the Science and Art Depart-
ment ; from their answers we learn that a large proportion
of them include in their palettes many pigments which
must be unhesitatingly condemned on account of their
want of stability. Thus no less than seventeen out of
the forty-six artists who responded to the appeal employ
three of the most fugitive pigments in the series —
namely, gamboge, brown madder, and indigo. Converted
into percentages, we may say that 37 out of 100
painters in water-colours use these three untrustworthy
pigments, besides others which are worse, and others
which are little better ; of course, they employ also
376
SOUTH KENSINGTON REPORT
certain colours as to the stability of which there is no
question. The following tabular statement gives the
proportion of artists, per loo, who use the eleven perish-
able pigments named below :
Gamboge ...
... 70
... Faded to 7
Indian yellow
.. 24
.. Faded to 6
Vermilion ...
.. 70
. . Gone black
Carmine ...
.. 8
.. Gone.
Crimson lake
.. 22
.. Gone.
Purple madder
.. 28
.. Faded to 8.
Brown madder
.. 74
. . Faded to 3
Brown pink
.. II
. . Faded to 7
Vandyke brown
•• 74
.. Gone.
Sepia
.. 65
.. Faded to 8.
Indigo
•• 52
. . Faded to 8.
I represents the
lightest tint, 8 the
darkest. The washes
of pigment were fully
exposed for twenty-
two months.
We call these pigments perishable with good reason.
For, according to the report under review, all of them were
found to have faded, materially and conspicuously, after
twenty-two months' full exposure in a south aspect, while
three of them had entirely disappeared, and another (ver-
milion) had become black. But this is not all. For under
a less severe trial (p. 45) — namely, exposure for the
same time, not to direct sunlight, but to a very bright
light from a window, ' under conditions approximating to
those to which pictures are usually subjected ' — six out
of the eleven pigments had faded, though in varying
degrees. With these figures and results before us, it is
impossible to resist the conclusion that a life of 100 years
is too much to allow to many of the water-colour drawings
of the present day. What shall we say, then, as to the
stability of the works of the earlier masters of the English
water-colour school ? How much care in the exclusion
of ' the more fugitive colours * was taken by the water-
colourists of 1780 to 1850 ? Could it be honestly said of
any large number of such works, in which gamboge
TRIALS OF WATER-COLOUR PAINTS 377
brown pink, crimson lake, sap-green, indigo, and sepia,
were generally employed without stint, ' that about a
century of exposure v/ould have to be given to water-
colour drawings in galleries lighted as are those at South
Kensington before any marked deterioration would be
visible in them '? * (Report, p. 46.)
Dr. Russell and Sir William Abney add, indeed, the
proviso, ' If painted with any but the more fugitive
colours.' But this condition cannot be said to have been
fulfilled by the works in question ; for in the great
majority of them, most of the six fugitive pigments which
we have just named were freely employed. And it is
these very pigments which have been proved by the
reporters themselves to suffer ' marked deterioration ' by
an exposure of twenty-one months only to strong daylight
without direct sunshine. Moreover, it must not be for-
gotten that the fading of a single important pigment in a
water-colour drawing is ruinous to the whole effect, de-
stroying the balance of the chromatic scheme of the artist
more effectually than a slight, but equal, degradation of
all the hues.
An instructive set of trials of water-colour paints was
commenced in May, 1894, ^.nd continued for four years
* In the preceding brief resume of certain parts of the South
Kensington Report no reference has been made to an argument,
developed in §§ x. to xv., in which it is contended that ' if a certain
tint be exposed to an intensity of radiation which we will call 100,
and which bleaches it in, say, i hour, then, if a similar tint be
exposed to an intensity i, it will require 100 hours' exposure to it
to effect the same bleaching.' The universal applicability of this
conclusion cannot be conceded by those who are familiar with
numerous instances in which no chemical or physical change
occurs when certain substances are exposed continuously for long
periods to a particular temperature, yet, when they are heated but
a degree or two higher, instantly alter, decompose, or react, as the
case may be.
378
TRIALS OF WATER-COLOUR PAINTS
by a sub-committee of the Burlington Fine Arts Club.
The results of these experiments have been embodied in
three reports. Winsor and Newton's moist water-colours
were used, flat washes on Whatman's * not hot pressed '
paper of the year 1888 being exposed to light in the
windows of the billiard-room of the club. One of the sets
so exposed was in an ordinary glazed frame, another set
was enclosed in hermetically sealed glass tubes containing
ordinary air ; while a third set was contained in similar
tubes, but the atmosphere present was maintained in a
dry state by means of a reservoir of burnt lime which
absorbed all or almost all traces of water in the pigments,
the paper, and the mount. The final comparisons were
made with parallel sets which had been kept in absolute
darkness. We give here, in tabular form, the condition
of the paints at the end of the fourth year of exposure :
reference must be made to the original reports for infor-
mation as to the results of six months' and of thirteen
months' exposure to sunshine.
RESULTS OF EXPOSURE FOR FOUR YEARS,
MAY 25, 1894, TO MAY 25, 1898
FROM
Pigment
Aureolin
Gamboge
Indian yellow
Cadmium yellow
Vermilion
Crimson lake ...
Rose madder ...
Indian red
Madder carmine
(Original depth = 10)
Set in Dry
Air
No change.
Faded to i.
Gone.
Deeper orange.
Greyish.
Faded to 0*5.
Faded to 8-5.
No change.
Faded to 9.
Set in Moist
Air
Faded to 9.
Faded to i.
Faded to i.
No change.
Greyish.
Gone.
Faded to 8*o
and purplish.
No change.
Darker, more
purplish.
Set in Frame
No change.
Faded to i.
Faded to i.
No change.
Greyish.
Gone.
Faded to 85
and purplish.
No change.
Darker, more
purplish.
TRIALS OF WATER-COLOUR PAINTS
379
Pigment
Set in Dry
Air
Set in Moist
Air
Set in Frame
Madder brown
Faded to 9.
Faded to 6,
Faded to 8,
purple gone.
purple gone.
Madder purple
Faded to 8.
Faded to 4.
Faded to 5.
Prussian blue ...
No change.
Faded to i.
Faded to 85.
French blue
No change.
No change.
No change.
Indigo
No change.
Faded to i,
Faded to i
greenish grey.
greenish grey.
Vandyke brown
Faded to 2, less
yellow.
Faded to i.
Faded to i.
Sepia
Faded to 8.
Faded to i .
Faded to 4.
Indigo with In-
dian red
No change.
Indigo gone.
Indigo gone.
The most striking results of these trials was the com-
plete stability of Prussian Blue and of Indigo when exposed
in air kept dry. The further remark may be made that
moist air, that is, ordinary air containing moisture and
confined in a sealed tube, inflicts more injury upon
alterable pigments than ordinary air enclosed in a frame
which does admit of some amount of ventilation occurring.
There is no difficulty in so constructing a frame as to
introduce a water-absorbing substance which may be
renewed from time to time. Thus we shall be able to
employ with confidence in water-colour painting madder
brown, madder purple, Prussian blue, indigo and sepia —
five pigments which under ordinary conditions of ex-
posure to sunshine suffer serious changes. But no method
has yet been devised by means of which we may safely
use gamboge, Indian yellow, vermilion, crimson lake and
Vandyke brown, although it must be stated that instances
have been recorded in which vermiUon as a water-colour
has stood very severe exposure-tests, especially when
white of egg has been mixed with it.
Amongst the series of trials of oil-paints made by the
author of this handbook, one set first arranged in 1880
38o EXPERIMENTS WITH OIL-PAINTS
may be described here. Chance's colourless plate-glass
was employed as the painting-ground, so as to avoid all
interference with the pigments from the surface on which
they were spread ; glass presents the further advantage
of permitting a complete examination of the back of each
specimen, and of changes in its translucency, opacity, or
texture. Each glass measured 8 inches by 6 ; the com-
plete series was prepared in duplicate — one for preserva-
tion in darkness, the other for exposure to all the light
that could be secured (in Kew) during live years in a
window facing nearly south-west. The majority of the
paints tried were obtained from four firms (Messrs. Winsor
and Newton, Messrs. Roberson and Co., M. Edouard of
Paris, and Schoenfeld of Diisseldorf). Specimens of
each pigment were reserved for further examination and
analysis. Some of the chief results obtained are given
in the annexed table; a few remarks on the changes
observed in some of the pigments which had been mixed
with flake white are added :
Pi<yment "^^^^'^ ^^
Residual Depth Change of Hue
figment Exposure
[Original-
: 10) and Remarks
Yellow ochre
5
10 .,
.. Browner; more translu-
cent.
Aureolin
5
9 ••
. None.
Indian yellow
5
8 ..
. Slighdy brownish.
Naples yellow (true)
5
10 ..
. None.
Pale yellow madder
2
7 ..
. Greyish salmon when
mixed with flake white.
Deep yellow madder
2
6 ..
. Dirty pink when mixed
with flake white.
Laque brun jaune ...
2
7 ..
. Lost much yellow.
Laque brun fonce ...
2
8 ..
. Lost much yellow.
Laque Robert, No. 5
2
2 ..
. Warm grey when mixed
with flake white.
Laque Robert, No. 6
2
4 ..
. Warm grey when mixed
with flake white.
Scarlet lake
5
7 ••
. Dull pinkish red.
TRIALS OF OIL-PAINTS
381
Years of
Exposure
5
2
Residual Depth
{Original =10)
I
10
9-5
9
8-5
8
10
Change of Hue
and Remarks
Almost gone.
None.
None.
Rather duller.
Slightly greener.
Slightly greener.
None.
Pigment
Crimson lake
Madder red ...
Madder carmine ... 5
Madder brown ... 2
Prussian blue ... 5
Indigo 5
Artificial ultramarine 5
A series of trials of seventeen madder colours in oil
was carried out in 1893. These were all prepared by
Lefranc of Paris. The samples were spread on thin
lantern glass 4 in. x 4 in., and when dry each glass was
cut in half. One half was exposed to sunshine from
March 26 until October 26, the other half of each specimen
being kept in darkness. At the end of the seven months
the halves of each sample were rejoined and carefully
mounted, and thus an instructive set of slides obtained.
The results are given in the following table, in which I
have grouped together those varieties of 'laque de garance'
which resembled one another in their degree of stability :
Name _
Laque de garance fonce
, , , , rose intense
,, ,, brun rouge
,, ,, rouge brun
Laque de garance rose
, , , , rose dore
brun pourpre ...
Laque de garance pourpre
,, ,, pourpre concentre
Carmin de garance ...
Laque de garance brun de madder
,, ,, brun
,, ,, brun fonce
,, ,, jaune capucine
,, ,, brun jaune
,, ,, nuance bitume
-Little or no change.
Slight change.
-1
Marked change.
Lost from 40 to 80 per
cent, of their original
depth.
382 TRIALS OF OIL-PAINTS
In the fourth or least stable group, comprising the
varieties of madder brown, it was noticeable that the
yellow constituent of the colour was most affected by
exposure, the bmn fonce and the nuance hitume losing all
their characteristic beauty of hue and becoming of a dull,
poor, rusty tint and not retaining over 20 per cent, of
their original depth. Even in Group II. the golden hue
of the rose doree was the only chromatic element of this
madder paint which had been lost to an appreciable
extent during the six months' exposure.
Experiments as to the degree of stability possessed by
many other oil-paints have been made ; the results will
be found for the most part incorporated with the accounts
given of the several pigments in Part III. One remark
may perhaps be usefully introduced in this place with
reference to the differences observable in the quality and
behaviour of pigments bearing the same name but ob-
tained from different artists' colourmen. It is a good
plan to place side by side on three trial-plates several
* makes ' of the same paint and to keep one set in dark-
ness, and to expose a second set to sunshine and a third
set to strong diffused daylight. It will sometimes be
found that the fascinating colour-quality v/hich at the
first glance recommends one sample is not preserved
after exposure, although the reverse experience is not
uncommon. It must not be forgotten that although the
composition and constituents of the vast majority of pig-
ments, both natural and artificial, are known, yet there
are differences in the methods of preparation which, in
some cases at least, are kept secret, and which result not
in differences of nuance only, but in differences of stability
also.
INDEX
Abney, Sir W,, 348, 371
Acetic acid, 85, 224
Acetone, 109
Actinic rays, 56, 348
Adjective pigments, 281
Albumen, 85
Alcohol, no
Alizarin, 194, 196
„ crimson, 194
„ lakes, 196
„ orang-e, 185
„ yellow, 1 85
Alterable pigments, 283 et seg.
Alum in paper, 12, 14
Alumina, hydrate, 50
. „ linoleate, 67, 190
„ oleate, 67, 190
Aluminium, sheet, 39, 41
Amber, 69
„ oil of, 71
„ varnish, 135
Amyl acetate, 355
„ alcohol, 112
Andrew, F. W., the late, 362
Anime, 73
Anti-vermilion, 189
Arabic acid, 92
Arnold paper, 9
Arrabida red, 204
Arsenic sulphides, 184
Asbestos, 26
„ in plaster, 22
Asphaltum, 260
Aureolin, 168
Aurora yellow, 166
Azurite, 249
Back, the, of pictures, 31, 38, 345
Balsams, 78
Barium chromate, 171
,, hydroxide, 104
,, sulphate, 148
Baryta- water, 104
„ white, 148
„ yellow, 171
Bassorin, 94
Beeswax, 79
Bell's medium, 140
Benzene, iii
Bistre, 259
Bitumen, 260
Black, ivory, 270
„ lead, 270
„ pigments, 264
Bleaching oil, 56
Bleu lumiere, 249
Blockz, J., 361
Blue, Antwerp, 240
„ black, 269
„ cobalt, 234
„ Egyptian, 250
„ indigo, 241
,, pigments, 226-251
,, Prussian, 236
„ Turnbull's, 238
,, verditer, 249
Brown, Caledonian, 256
„ Cappagh, 259
,, pigments, 252-263
,, pink, 181
„ Vandyke, 257
Burlington Club trials, 378
Burnt carmine, 209
„ lime, 19
,, sienna, 256
,, umber, 254
Burton's cobalt, 235
,, potters' pink, 200
383
384
INDEX
Cadmium, green, 225
„ pale, 162
„ red, 166
,, versus chrome, 183
,, yellow, 162
Calcium carbonate, 19, 23
,, hydroxide, 19
„ oxide, 19
,, silicate, 20, 304
„ sulphate, 23, 27, 307, 35^
Caledonian brown, 256
Camphor, 119
Canada balsam, 79
Canvas, 34
,, back of, 37
,, preservation of, 36-39
,, priming of, 34
,, Willesden, 41
Cappagh brown, 258
Carbon bisulphide, 108
Carbonate of copper, 221, 249
„ lead, 145
lime, 19
Carbonates, 277, 279
Carmine, 207
Cassel brown, 25S
Cellulose, 11
Ceramic pigments, 200, 218, 235
Ceresin, 81
Cerulean blue or cerulium, 235
Chafing varnish, 353
Charcoal blacky 268
Chessylite, 249
Chinese ink, 265
„ vermilion, 189
„ white, 152
Chloroform, 109
Chromates, 279
Chrome green, 214, 216
„ yellow, 182
Chromium, green oxide of, 214
Cineol, 107, 123
Cinnabar, 187
Citral, 123
Coagulation of albumen, 85
Cobalt blue, 234
„ „ Burton's, 235
„ green, 218
„ resinate, 57
„ violet, 211
,, yellow, 1 68
Cobaltinitrites, 168
Coccus cacti, 207
„ ilicis, 207
Coccus lacca, 205
Cochineal lake, 207
CcEruleum, 235
Collodion, 37
Conservation of pictures, 38^/ seq., 342
Copal, Angola, 73
„ Benguela, 73
,, Kauri, 74
„ oil of, 139
„ oil-varnish, 135
,, pebble, 72
,, Sierra Leone, 72
,, spirit varnish, 133
,, West Indian, 74
,, Zanzibar, 73
Copals compared, jj
Cotton paper, 11
Cowdi resin, 74, 138
Cremnitz white, 148
Crimson lake, 208
Cyanine, 240
Cymene, 124
Cyprusite, 161
Dammar, 74
Dextrin, 97
Dextrose, 97
Diluents, 106-124
Dipentene, 118
Distilled water, 106
Dryers, 55, 57, 125
Egg-medium, 85
Egyptian blue, 250
Elements, 275, 276, 278
Elemi, 142
Emerald green, 219
Emerald oxide of chromium, 216
Emulsions, 87
Epichlorhydrin, 11 1
Esparto, 12
Ether, 108
Ethereal hydrogen peroxide, 17, 151
Eucalyptol, 123
Eucalyptus oils, 122
Ferrocyanides, 236
Flake white, 145-15 1, 328
„ action on oil, 64, 32S
Freeman's white, 151
Fresco grounds, 21, 22.
,, method, 303
,, secco, 22, 305
Frescoes, Asiatic, 28, 307
INDEX
385
Frescoes, protected and restored, 356
Fugitive pigments, 2S4, 287, 2S9
Galleries, picture, 38, 342, 349
Gambier Parry's medium, 142, 315
Gamboge, 172
Gelatin, 10, 88
Geraniol. 107
Gesso, 32
Glass, protective, 348
Glue, 35. 88
Glycerides, 51
Glycerin, 51, 98
Gold-point, 322
Graphite, 270
Green, chrome, 214
,, madder, 285
„ oxide of chromium, 214
„ ultramarine, 232
,, verditer, 221
Gum arable, 91
„ British, 97
„ Cape, 94
„ Senegal, 92
„ Suakim, 92
„ tragacanth, 93
Gypsum, 22, 23, 27
Hartley, the late Sir W. N., 366
Hellige's colorimeter, 363
Hodgkinson paper, 9
Honey, 97
Hydrate of alumina, 50, 194, 208
Hydrates of copper, 249
„ iron, 157, 202
Hydrocarbons, 81, in, 11 2-1 17
Hydrogen peroxide, 35, 151
Hydroxides or hydrates, 279
Illuminated manuscripts, 190, 331
Indian ink, 264
„ lake, 205
,, red, 202
,, yellow, 174
Indigo, 241
„ brominated, 247
„ synthetic, 247
Infusorial earth, 16
Ink, Indian, 264
Intonaco, 21
Iron, compounds of, 157, 177, 200-
205
Ivory, 19
„ black, 270
Kauri, 74, 138
Kermes lake, 207
Kieselguhr, 16
Kings' yellow, 184
Kowdi, 74, 138
Lsevulose, 97
Lamp-black, 267
Lapis lazuli, 226
Laurie, Prof. A, P., viii, 22, 250, 313
Lavender, oil of, 122
Lead antimoniate, 178
,, chromate, 182
„ dryers, 55, 59
,, oxychloride, 152
,, ■ red, 210
„ sugar of, 125
„ sulphate, 151
„ white, 145-15 1
Lecithin in egg-yolk, 87
Leitch's blue, 240
Lefranc's madders, 381
Lemon, oil of, 123
„ yellow, 171
Light, action of, 56, 342-349, 358-382
,, red, 200
Lime, burnt, 19, 20
„ carbonate, 18
„ caustic, 19, 22
„ hydrate, 19
,, mild, 19
„ -putty, 20
,, slaked, 19
„ sulphate, 23, 27
,, -water, 19, 102
Limonene, 118, 123
Linen fibre, 7, 11
Linoleic acid and linolein, 51
Linoleum, 40
Linoxine, 51
Linseed, 47
Linseed oil, 47-51
„ „ siccative, 57
,, „ testing, 60
,, white, 48
Litmus, 13
Lovibond's tintometer, 359
Madder brown, 196, 198
„ lakes, 194
„ rose, 198
Malachite, 221
Manganese borate, 57
„ dryers, sj
25
386
INDEX
Manganese oil, 58
Oils,d
rying, 46
„ resinate, 57
„ fixed, 45
„ violet, 251
Old paintings, 325-341
Manuscripts, illuminated, 190, 331
Orang
e, oil of, 123
Marouflage, 26
Organic pigments, 277, 2S0
Mars violet, 177, 203
Orpiment, 184
„ yellow, 177
Ossein
, 16
Mastic, 76
Ostwald, Dr. W., notes by, 37-40, 52-
„ varnish, 131
^55>
J 1 9-32 1
Mediums, 139-142
0. W
paper, 9
Megilp, 140
Oxides, 276, 278
Mercury lamp, 56
Methylated spirit, 109
Pamters' materials, i
Mineral lake, 199
Painting methods, 301
,, pink, 200
,,
,, Central Asian, 28,
„ violet, 251
307
Mixed varnishes, 130
,,
„ distemper, 22, 301
Mummy, 262
,,
,, fresco, 21, 304
>>
„ Gambler Parry, 25,
Naphthalene as resin-solvent, 13S
309
Naples yellow, 178
,,
oil, 309
National Gallery, 331
pastel, 318-321
„ British Art, 335
j>
,, stereochromy, 24,
„ „ Portrait, 336
307
Newman's colours, 370
„
,, tempera, 22, 301
Nut-oil, 62
"
water-colour, 317,
362
Ochre, chrome in, 161
»
, , water-glass, 24, 307
„ Perigord, 159
Panels
,29
,, red, 204
})
preparation of, 30
,, yellow, 157
Paper,
7 .,.
Oil, action on paints, 63
>)
acid in, 13, 369
,, amber, 119
>■>
alum in, 10, 13, 14
„ copal, 119
>>
analyses of, 9
„ drying, 46
,,
ash of, 10
,, eucalyptus, 122
,,
cotton, II
,, extraction, 47
5>
fibre in, 1 1
,, in egg-yolk, 87
,,
linen, 9, 1 1
,, m paints, 65
„
size in, 10, 14
,, lavender, 122
,,
sugar, 17
,, lemon, 123
,,
testing, 12
,, linseed, 47-51
,,
Turner, 17
„ manganese, 58
,,
Varley, 17
„ nut, 62
,,
water in, 9
„ orange, 123
,,
wood-pulp in, 12, 17
,, painting, 309
Paraffi
n, 81, 120
,, poppy, 61
>>
copal medium, 141
„ rosemary, 123
,,
on frescoes, 357
„ semi-drying, 46
>•>
paste, 356
,, siccative, 46, 57
Parchment, 16
,, turpentine, 112
Pastel painting, 318-321
,, tung, 63
Payne-
s grey, 273
,, varnishes, 135
Perigord, orange and raw, 159
Oils, essential, 122
Permanent pigments, 284-288
INDEX
387
Peroxide of hydrogen, 150, 338
Petroleum spirit, 120
Phellandrene, 117
Pigments classified, 274
,, interacting, 275
Pinene, X17
Piuri, 174
Plaster, 18-28
Platinum-point, 322
Poppy oil, 6 1
Potters' pink, 200
Primrose yellow, 172
Prussian blue, 236
,, brown, 263
Punicin, 247
Purple, Tyrian, 247, 334
Purpurin, 194
Queen's House, Greenwich, 358
Raw sienna, 254
„ umber, 253
Red lead, 210
,, ochre, 204
,, pigments, 186-21 1
Resin, 68
„ amber, 69
„ anime, 73, 74
„ copal, 72-77
„ cowdi, 74
,, dammar, -js
,, kauri, 74
,, mastic, 76
,, sandarac, 75
Resinates, 55, 59, 69, 113
Restoration of pictures, 351
Restricted palettes, 294-298
Reynolds, Sir Joshua, 292, 338, 361
Roberson's medium, 140
Rock crystal to be avoided, 349
Rood, Prof. O. N., 365
Rose madder, 198
Rosemary, oil of, 123
Rosin, 113
Rosinates, 55, 59, 113
Royal Exchange panels, 26
Ruskin, John, 347
Russell, Dr. W. J., the late, 371, 373
Sandarac, 75
„ varnish, 76
Scheele's green, 221
Schweinfurt green, 219
Selected palettes, 290
Sepia, 272
Siccatives, 55, 125-129
Sienna, burnt, 256
„ raw, 254
Silicates, 277, 279
„ alkaline, 100, 307
Silver-point method, 321
Simpson, W., the late, 369
Size, 35, 88
Size-painting, 303, 332
Slate, 27
Slate-grey, 273
Smalt, 248
Soane Museum, 151, 338
Solvents, 106-124
South Kensington Museum, 338, 348
y> „ Report, 371
Spike, oil of, 122
Spirit-fresco grounds, 25
„ medium, 142
„ method, 142, 315
Starch, 94
„ soluble, 95
Stein, Sir Aurel, 8, 28, 307
Stereochromy, 24, loi, 307
Stone, 18, 27
Strasburg turpentine, 78
Straw in paper, 12
Substantive pigments, 281
Sugar-paper, 17
Sulphides, 276, 278
Sylvestrene, 118
Taylor, Mr. J. Scott, viii, 218
Tempera-grounds, 22
„ method, 301
Terpenes, 112-123
Terre verte, 212
„ „ adulterated, 214
Testing paper, 12
„ pigments, xi
„ turpentine, 115
Toluene, 111
Turnbull's blue, 238
Turner paper, 17
Turpentine, 112
„ oil of, 116
„ Strasburg, 78
„ Venice, 78
Turpentines, 112
Ultramarine, 226
„ artificial, 229
ash, 227
388 INDEX
Ultramarine, ffreen, 232
„ lilac, 232
„ red, 232
„ testing, 233
„ violet 232
Ultra-violet rays, 56, 318
Umber, 252
Vanadium yellow, 183
Varley paper, 1 7
Varnish, amber, 133, 135
„ copal, 133-140
„ mastic, 131
oil, 131, 135
,, sandarac, 133
„ spirit, 131, 133
Varnishes, 130-139
Vellum, 16
Venetian red, 201
Venice turpentine, 78
Verdig-ris, 223
Verditer, 221, 249
Vermilion, artificial, 187
„ instability of, 190
„ native, 186
„ tested, 189
Vernalis, 218
Victoria and Albert Museum, 338,
348
Vine black, 269
Violet, cobalt, 211
„ manganese, 251
,, ultramarine, 232
Viridian, 216
Viscose, 14
Wallace Collection, 337
Ward, Mr. James, 357
Water, 106
„ baryta-, 18, 104, 304
Water in drawings, 345
„ in paper, 9, 317
„ in resms, 69
„ lime-, 19, 102, 304
Water-colour method, 317
Water-glass, 100
Wax, bees', 79
„ Brazilian, 80
„ Ceresin, 81
„ Chinese, 80
,, Japanese, 81
,, -painting, 80
„ paraffin, 81
Whatman paper, 9, 12, 371, 378
White-lead, 145
„ „ action on oil, 64
„ „ adulteration of, 147
„ ,, bleaching discoloured, 150
„ „ defects of, 149
„ „ impurities in, 147
White of egg, 85
White pigments, 145-156
Willesden canvas, 41
Wood, 29-33
„ pulp, mechanical, 17
„ spirit, 109
Yellow, cobalt, 168
„ kings', 184
„ lake, 180
„ madder, 181, 185
„ ochre, 157
„ pigments, 157-185
Zinc borate, 128
„ chromate, 172
„ oxide, 152
„ sulphate, 129
„ sulphide, 154
„ white, 152
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